Method for transmitting/receiving d2d signal in wireless communication system and apparatus therefor

ABSTRACT

The present invention relates to a method and apparatus for transmitting a device-to-device (D2D) discovery by a first terminal having a single TX chain in a wireless communication system supporting carrier aggregation (CA). Specifically, the present invention comprises the steps of: setting a particular gap for a first resource area related to D2D signal transmission/reception on a primary cell (PCell); and transmitting a D2D discovery through a non-primary cell (non-PCell) in a time interval corresponding to the particular gap, wherein the particular gap is a time interval set for allowing the single TX chain to cover a switching operation between the PCell and the non-PCell, and a transmission resource for the D2D discovery is determined according to the type of D2D discovery.

TECHNICAL FIELD

The present disclosure relates to a wireless communication system, and more particularly, to a method and apparatus for transmitting/receiving a device-to-device (D2D) signal in a wireless communication system.

BACKGROUND ART

A 3rd generation partnership project long term evolution (3GPP LTE) (hereinafter, referred to as ‘LTE’) communication system which is an example of a wireless communication system to which the present invention can be applied will be described in brief.

FIG. 1 is a diagram illustrating a network structure of an evolved universal mobile telecommunications system (E-UMTS) which is an example of a wireless communication system. The E-UMTS is an evolved version of the conventional UMTS, and its basic standardization is in progress under the 3rd generation partnership project (3GPP). The E-UMTS may be referred to as a long term evolution (LTE) system. Details of the technical specifications of the UMTS and E-UMTS may be understood with reference to Release 7 and Release 8 of “3rd Generation Partnership Project; Technical Specification Group Radio Access Network”.

Referring to FIG. 1, the E-UMTS includes a user equipment (UE), base stations (eNode B; eNB), and an access gateway (AG) which is located at an end of a network (E-UTRAN) and connected to an external network. The base stations may simultaneously transmit multiple data streams for a broadcast service, a multicast service and/or a unicast service.

One or more cells exist for one base station. One cell is set to one of bandwidths of 1.44, 3, 5, 10, 15 and 20 MHz to provide a downlink or uplink transport service to several user equipments (UEs). Different cells may be set to provide different bandwidths. Also, one base station controls data transmission and reception for a plurality of UEs. The base station transmits downlink (DL) scheduling information of downlink data to the corresponding UE to notify the corresponding UE of time and frequency domains to which data will be transmitted and information related to encoding, data size, and hybrid automatic repeat and request (HARQ). Also, the base station transmits uplink (UL) scheduling information of uplink data to the corresponding UE to notify the corresponding UE of time and frequency domains that can be used by the corresponding UE, and information related to encoding, data size, and HARQ. An interface for transmitting user traffic or control traffic may be used between the base stations. A core network (CN) may include the AG and a network node or the like for user registration of the UE. The AG manages mobility of the UE on a tracking area (TA) basis, wherein one TA includes a plurality of cells.

Although the wireless communication technology developed based on WCDMA has been evolved into LTE, request and expectation of users and providers have continued to increase. Also, since another wireless access technology is being continuously developed, new evolution of the wireless communication technology will be required for competitiveness in the future. In this respect, reduction of cost per bit, increase of available service, use of adaptable frequency band, simple structure and open type interface, proper power consumption of the UE, etc. are required.

DISCLOSURE Technical Problem

An aspect of the present disclosure devised to solve the conventional problem is to provide a method and apparatus for transmitting/receiving a device-to-device (D2D) signal in a wireless communication system.

It will be appreciated by persons skilled in the art that the objects that could be achieved with the present disclosure are not limited to what has been particularly described hereinabove and the above and other objects that the present disclosure could achieve will be more clearly understood from the following detailed description.

Technical Solution

In an aspect of the present disclosure, a method for transmitting a device-to-device (D2D) discovery signal by a first user equipment (UE) having a single transmission (TX) chain in a wireless communication system supporting carrier aggregation (CA) includes configuring a predetermined gap for a first resource area related to transmission and reception of a D2D signal on a primary cell (PCell), and transmitting a D2D discovery signal in a non-primary cell (non-PCell) in a time period corresponding to the predetermined gap. The predetermined gap is configured to cover switching between the PCell and the non-PCell of the single TX chain, and transmission resources for the D2D discovery signal are determined according to the type of the D2D discovery signal.

If the type of the D2D discovery signal is type 1 discovery, the transmission resources for the D2D discovery signal may be determined only to be radio resources of the time period corresponding to the predetermined gap.

If the type of the D2D discovery signal is type 1 discovery, and the transmission resources for the D2D discovery signal are radio resources which do not belong to the time period corresponding to the predetermined gap, the transmission of the D2D discovery signal may not be performed.

If the type of the D2D discovery signal is type 1 discovery, and the transmission resources for the D2D discovery signal are radio resources which do not belong to the time period corresponding to the predetermined gap, the transmission resources for the D2D discovery signal may be repeatedly reconfigured until the transmission resources for the D2D discovery signal are determined to be radio resources of the time period corresponding to the predetermined gap.

If the type of the D2D discovery signal is type 2 discovery, the transmission resources for the D2D discovery signal may be radio resources of a second resource area signaled by a base station (BS).

In another aspect of the present disclosure, a first UE having a single TX chain, for transmitting a D2D discovery signal in a wireless communication system supporting CA includes a radio frequency unit, and a processor. The processor is configured to configure a predetermined gap for a first resource area related to transmission and reception of a D2D signal on a primary cell (PCell), and transmit a D2D discovery signal in a non-primary cell (non-PCell) in a time period corresponding to the predetermined gap. The predetermined gap is configured to cover switching between the PCell and the non-PCell of the single TX chain, and transmission resources for the D2D discovery signal are determined according to the type of the D2D discovery signal.

Advantageous Effects

According to an embodiment of the present disclosure, a device-to-device (D2D) signal can be transmitted/received efficiently in a wireless communication system.

It will be appreciated by persons skilled in the art that the effects that can be achieved with the present disclosure are not limited to what has been particularly described hereinabove and other advantages of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the disclosure and are incorporated in and constitute a part of this application, illustrate embodiments of the disclosure and together with the description serve to explain the principle of the disclosure. In the drawings:

FIG. 1 is an exemplary view illustrating a network structure of an evolved universal mobile telecommunications system (E-UMTS) as an exemplary wireless communication system;

FIG. 2 is a view illustrating the architecture of a control plane and a user plane of a radio interface protocol between a user equipment (UE) and an evolved UMTS terrestrial radio access network (E-UTRAN) conforming to a 3^(rd) generation partnership project (3GPP) radio access network standard;

FIG. 3 is an exemplary view illustrating physical channels used in a 3GPP system and a general signal transmission method using the physical channels;

FIG. 4 is an exemplary view illustrating the structure of a radio frame used in a long term evolution (LTE) system;

FIG. 5 is an exemplary view illustrating a resource grid for the duration of a downlink (DL) slot;

FIG. 6 is an exemplary view illustrating the structure of a DL subframe;

FIG. 7 is a view illustrating the structure of an uplink (UL) subframe;

FIG. 8 is a view referred to for describing device-to-device (D2D) communication;

FIG. 9 is a view referred to for describing a

exemplary resource unit (RU) configuration for D2D communication;

FIG. 10 is a view illustrating periodic occurrences of a resource pool related to a discovery message;

FIG. 11 is a view referred to for describing configuration of a D2D synchronization signal (D2DSS) subframe (SF) and a D2DSS relay SF for an in-coverage UE and an out-of-coverage UE;

FIG. 12 is a view illustrating the positions of resource pools in which a D2DSS is transmitted;

FIG. 13 is a view referred to for describing options related to the present disclosure;

FIG. 14 is a view referred to for comparing DL gaps required for neighbor cells having synchronization window lengths w2 and w1;

FIG. 17 is a view illustrating coverage of multiple carriers related to the present disclosure;

FIG. 16 is a view illustrating a heterogeneous network to which multiple carriers are applied; and

FIG. 17 is a block diagram of a base station (BS) and a UE which are applicable to an embodiment of the present disclosure.

BEST MODE FOR CARRYING OUT THE INVENTION

The following technology may be used for various wireless access technologies such as code division multiple access (CDMA), frequency division multiple access (FDMA), time division multiple access (TDMA), orthogonal frequency division multiple access (OFDMA), and single carrier frequency division multiple access (SC-FDMA). The CDMA may be implemented by the radio technology such as universal terrestrial radio access (UTRA) or CDMA2000. The TDMA may be implemented by the radio technology such as global system for mobile communications (GSM)/general packet radio service (GPRS)/enhanced data rates for GSM evolution (EDGE). The OFDMA may be implemented by the radio technology such as IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, and evolved UTRA (E-UTRA). The UTRA is a part of a universal mobile telecommunications system (UMTS). A 3rd generation partnership project long term evolution (3GPP LTE) is a part of an evolved UMTS (E-UMTS) that uses E-UTRA, and adopts OFDMA in a downlink (DL) and SC-FDMA in an uplink (UL). LTE-advanced (LTE-A) is an evolved version of the 3GPP LTE.

For clarification of the description, although the following embodiments will be described based on the 3GPP LTE/LTE-A, it is to be understood that the technical spirits of the present invention are not limited to the 3GPP LTE/LTE-A. Also, specific terminologies hereinafter used in the embodiments of the present invention are provided to assist understanding of the present invention, and various modifications may be made in the specific terminologies within the range that they do not depart from technical spirits of the present invention.

FIG. 2 is a diagram illustrating structures of a control plane and a user plane of a radio interface protocol between a user equipment (UE) and E-UTRAN based on the 3GPP radio access network standard. The control plane means a passageway where control messages are transmitted, wherein the control messages are used by the UE and the network to manage call. The user plane means a passageway where data generated in an application layer, for example, voice data or Internet packet data are transmitted.

A physical layer as the first layer provides an information transfer service to an upper layer using a physical channel The physical layer is connected to a medium access control (MAC) layer via a transport channel, wherein the medium access control layer is located above the physical layer. Data are transferred between the medium access control layer and the physical layer via the transport channel. Data are transferred between one physical layer of a transmitting side and the other physical layer of a receiving side via the physical channel. The physical channel uses time and frequency as radio resources. In more detail, the physical channel is modulated in accordance with an orthogonal frequency division multiple access (OFDMA) scheme in a DL, and is modulated in accordance with a single carrier frequency division multiple access (SC-FDMA) scheme in an uplink.

A medium access control (MAC) layer of the second layer provides a service to a radio link control (RLC) layer above the MAC layer via a logical channel. The RLC layer of the second layer supports reliable data transmission. The RLC layer may be implemented as a functional block inside the MAC layer. In order to effectively transmit data using IP packets such as IPv4 or IPv6 within a radio interface having a narrow bandwidth, a packet data convergence protocol (PDCP) layer of the second layer performs header compression to reduce the size of unnecessary control information.

A radio resource control (RRC) layer located on the lowest part of the third layer is defined in the control plane only. The RRC layer is associated with configuration, re-configuration and release of radio bearers (‘RBs’) to be in charge of controlling the logical, transport and physical channels. In this case, the RB means a service provided by the second layer for the data transfer between the UE and the network. To this end, the RRC layers of the UE and the network exchange RRC message with each other. If the RRC layer of the UE is RRC connected with the RRC layer of the network, the UE is in an RRC connected mode. If not so, the UE is in an RRC idle mode. A non-access stratum (NAS) layer located above the RRC layer performs functions such as session management and mobility management.

One cell constituting a base station eNB is set to one of bandwidths of 1.4, 3.5, 5, 10, 15, and 20 MHz and provides a DL or UL transmission service to several UEs. At this time, different cells may be set to provide different bandwidths.

As DL transport channels carrying data from the network to the UE, there are provided a broadcast channel (BCH) carrying system information, a paging channel (PCH) carrying paging message, and a DL shared channel (SCH) carrying user traffic or control messages. Traffic or control messages of a DL multicast or broadcast service may be transmitted via the DL SCH or an additional DL multicast channel (MCH). Meanwhile, as UL transport channels carrying data from the UE to the network, there are provided a random access channel (RACH) carrying an initial control message and an UL shared channel (UL-SCH) carrying user traffic or control message. As logical channels located above the transport channels and mapped with the transport channels, there are provided a broadcast control channel (BCCH), a paging control channel (PCCH), a common control channel (CCCH), a multicast control channel (MCCH), and a multicast traffic channel (MTCH).

FIG. 3 is a diagram illustrating physical channels used in a 3GPP LTE system and a general method for transmitting a signal using the physical channels.

The UE performs initial cell search such as synchronizing with the base station when it newly enters a cell or the power is turned on at step S301. To this end, the UE synchronizes with the base station by receiving a primary synchronization channel (P-SCH) and a secondary synchronization channel (S-SCH) from the base station, and acquires information such as cell ID, etc. Afterwards, the UE may acquire broadcast information within the cell by receiving a physical broadcast channel (PBCH) from the base station. Meanwhile, the UE may identify a DL channel status by receiving a DL reference signal (DL RS) at the initial cell search step.

The UE which has finished the initial cell search may acquire more detailed system information by receiving a physical DL shared channel (PDSCH) in accordance with a physical DL control channel (PDCCH) and information carried in the PDCCH at step S302.

Afterwards, the UE may perform a random access procedure (RACH) such as steps S303 to S306 to complete access to the base station. To this end, the UE may transmit a preamble through a physical random access channel (PRACH) (S303), and may receive a response message to the preamble through the PDCCH and the PDSCH corresponding to the PDCCH (S304). In case of a contention based RACH, the UE may perform a contention resolution procedure such as transmission (S305) of additional physical random access channel and reception (S306) of the physical DL control channel and the physical DL shared channel corresponding to the physical DL control channel

The UE which has performed the aforementioned steps may receive the physical DL control channel (PDCCH)/physical DL shared channel (PDSCH) (S307) and transmit a physical UL shared channel (PUSCH) and a physical UL control channel (PUCCH) (S308), as a general procedure of transmitting UL/DL signals. Control information transmitted from the UE to the base station will be referred to as UL control information (UCI). The UCI includes hybrid automatic repeat and request acknowledgement/negative-ack (HARQ ACK/NACK), scheduling request (SR), channel state information (CSI), etc. In this specification, the HARQ ACK/NACK will be referred to as HARQ-ACK or ACK/NACK (A/N). The HARQ-ACK includes at least one of positive ACK (simply, referred to as ACK), negative ACK (NACK), DTX and NACK/DTX. The CSI includes channel quality indicator (CQI), precoding matrix indicator (PMI), rank indication (RI), etc. Although the UCI is generally transmitted through the PUCCH, it may be transmitted through the PUSCH if control information and traffic data should be transmitted at the same time. Also, the UE may non-periodically transmit the UCI through the PUSCH in accordance with request/command of the network.

FIG. 4 is a diagram illustrating a structure of a radio frame used in an LTE system.

Referring to FIG. 4, in a cellular OFDM radio packet communication system, UL/DL data packet transmission is performed in a unit of subframe, wherein one subframe is defined by a given time interval that includes a plurality of OFDM symbols. The 3GPP LTE standard supports a type 1 radio frame structure applicable to frequency division duplex (FDD) and a type 2 radio frame structure applicable to time division duplex (TDD).

FIG. 4(a) is a diagram illustrating a structure of a type 1 radio frame. The DL radio frame includes 10 subframes, each of which includes two slots in a time domain. A time required to transmit one subframe will be referred to as a transmission time interval (TTI). For example, one subframe may have a length of 1 ms, and one slot may have a length of 0.5 ms. One slot includes a plurality of OFDM symbols in a time domain and a plurality of resource blocks (RB) in a frequency domain. Since the 3GPP LTE system uses OFDM in a DL, OFDM symbols represent one symbol interval. The OFDM symbol may be referred to as SC-FDMA symbol or symbol interval. The resource block (RB) as a resource allocation unit may include a plurality of continuous subcarriers in one slot.

The number of OFDM symbols included in one slot may be varied depending on configuration of a cyclic prefix (CP). Examples of the CP include an extended CP and a normal CP. For example, if the OFDM symbols are configured by the normal CP, the number of OFDM symbols included in one slot may be 7. If the OFDM symbols are configured by the extended CP, since the length of one OFDM symbol is increased, the number of OFDM symbols included in one slot is smaller than that of OFDM symbols in case of the normal CP. For example, in case of the extended CP, the number of OFDM symbols included in one slot may be 6. If a channel state is unstable like the case where the UE moves at high speed, the extended CP may be used to reduce inter-symbol interference.

If the normal CP is used, since one slot includes seven OFDM symbols, one subframe includes 14 OFDM symbols. At this time, first maximum three OFDM symbols of each subframe may be allocated to a physical DL control channel (PDCCH), and the other OFDM symbols may be allocated to a physical DL shared channel (PDSCH).

FIG. 4(b) illustrates the structure of a type-2 radio frame. The type-2 radio frame includes two half frames, each of which has 4 normal subframes including 2 slots and a special subframe including a downlink pilot time slot (DwPTS), a guard period (GP), and an uplink pilot time slot (UpPTS).

In the special subframe, the DwPTS is used for initial cell search, synchronization, or channel estimation on a UE. The UpPTS is used for channel estimation and acquisition of uplink transmission synchronization for a UE in an eNB. That is, the DwPTS is used for downlink transmission, and the UpPTS is used for uplink transmission. In particular, the UpPTS is utilized for a PRACH preamble or SRS transmission. In addition, the GP is a period between uplink and downlink, which is intended to eliminate uplink interference caused by multipath delay of a downlink signal.

The current 3GPP standard document defines configuration of the special subframe as shown in Table 1 below. Table 1 shows DwPTS and UpPTS given when T_(s)=1/(15000×2048), and the other region is configured as a GP.

TABLE 1 Normal cyclic prefix in downlink UpPTS Extended cyclic prefix in downlink Normal Extended UpPTS Special subframe cyclic prefix cyclic prefix Normal cyclic Extended cyclic configuration DwPTS in uplink in uplink DwPTS prefix in uplink prefix in uplink 0 6592 · T_(s) 2192 · T_(s) 2560 · T_(s)  7680 · T_(s) 2192 · T_(s) 2560 · T_(s) 1 19760 · T_(s) 20480 · T_(s) 2 21952 · T_(s) 23040 · T_(s) 3 24144 · T_(s) 25600 · T_(s) 4 26336 · T_(s)  7680 · T_(s) 4384 · T_(s) 5120 · T_(s) 5  6592 · T_(s) 4384 · T_(s) 5120 · T_(s) 20480 · T_(s) 6 19760 · T_(s) 23040 · T_(s) 7 21952 · T_(s) 12800 · T_(s) 8 24144 · T_(s) — — — 9 13168 · T_(s) — — —

In the TDD system, the structures of the type-2 radio subframe, namely uplink/downlink subframe configurations (UL/DL configurations), are given as shown in [Table 2] below.

TABLE 2 Downlink- to-Uplink Uplink- Switch- downlink point Subframe number configuration periodicity 0 1 2 3 4 5 6 7 8 9 0 5 ms D S U U U D S U U U 1 5 ms D S U U D D S U U D 2 5 ms D S U D D D S U D D 3 10 ms  D S U U U D D D D D 4 10 ms  D S U U D D D D D D 5 10 ms  D S U D D D D D D D 6 5 ms D S U U U D S U U D

In [Table 2], D denotes a downlink subframe, U denotes an uplink subframe, and S denotes the special subframe. Table 2 also shows downlink-to-uplink switch-point periodicity in uplink/downlink subframe configuration of each system.

The illustrated radio frame structures are merely illustrative, and various modifications may be made to the number of subframes included in a radio frame, the number of slots included in a subframe, or the number of symbols included in a slot.

FIG. 5 is a diagram illustrating a resource grid of a downlink slot.

Referring to FIG. 5, the downlink slot includes a plurality of N_(symb) ^(DL) OFDM symbols in a time domain and a plurality of N_(RB) ^(DL) resource blocks in a frequency domain. Since each resource block includes N_(sc) ^(RB) subcarriers, the downlink slot includes N_(RB) ^(DL)×N_(sc) ^(RB) subcarriers in the frequency domain. Although FIG. 5 illustrates that the downlink slot includes seven OFDM symbols and the resource block includes twelve subcarriers, it is to be understood that the downlink slot and the resource block are not limited to the example of FIG. 5. For example, the number of OFDM symbols included in the downlink slot may be varied depending on the length of the CP.

Each element on the resource grid will be referred to as a resource element (RE). One resource element is indicated by one OFDM symbol index and one subcarrier index. One RB includes symb N_(symb) ^(DL)×N_(sc) ^(RB) number of resource elements. The number RB of resource blocks included in the downlink slot depends on a downlink transmission bandwidth configured in the cell.

FIG. 6 is a diagram illustrating a structure of a DL subframe.

Referring to FIG. 6, a maximum of three (four) OFDM symbols located at the front of the first slot of the subframe correspond to a control region to which a control channel is allocated. The other OFDM symbols correspond to a data region to which a physical downlink shared channel (PDSCH) is allocated. Examples of downlink control channels used in the LTE system include a physical control format indicator channel (PCFICH), a physical downlink control channel (PDCCH), and a physical hybrid ARQ indicator channel (PHICH). The PCFICH is transmitted from the first OFDM symbol of the subframe, and carries information on the number of OFDM symbols used for transmission of the control channel within the subframe. The PHICH carries hybrid automatic repeat request acknowledgement/negative-acknowledgement (HARQ ACK/NACK) signals in response to UL transmission.

The control information transmitted through the PDCCH will be referred to as downlink control information (DCI). The DCI includes resource allocation information for a user equipment or user equipment group. For example, the DCI includes uplink/downlink scheduling information, an uplink transmission (Tx) power control command, etc.

The PDCCH may include transport format and resource allocation information of a downlink shared channel (DL-SCH), transport format and resource allocation information of an uplink shared channel (UL-SCH), paging information on a paging channel (PCH), system information on the DL-SCH, resource allocation information of an upper layer control message such as a random access response transmitted on the PDSCH, a set of Tx power control commands of individual UEs within a random user equipment group, Tx power control command, and activity indication information of voice over Internet protocol (VoIP). A plurality of PDCCHs may be transmitted within the control region. The UE may monitor the plurality of PDCCHs. The PDCCH is transmitted on an aggregate of one or a plurality of continuous control channel elements (CCEs). The CCE is a logic allocation unit used to provide the PDCCH with a coding rate based on the status of a radio channel The CCE corresponds to a plurality of resource element groups (REGs). The format of the PDCCH and the number of available bits of the PDCCH are determined depending on the number of CCEs. The base station determines a PDCCH format depending on the DCI which will be transmitted to the user equipment, and attaches a cyclic redundancy check (CRC) to the control information. The CRC is masked with an identifier (for example, a radio network temporary identifier (RNTI)) depending on usage of the PDCCH or owner of the PDCCH. For example, if the PDCCH is for a specific user equipment, the CRC may be masked with cell-RNTI (C-RNTI) of the corresponding user equipment. If the PDCCH is for a paging message, the CRC may be masked with a paging identifier (for example, paging-RNTI (P-RNTI)). If the PDCCH is for system information (in more detail, a system information block (SIB)), the CRC may be masked with system information RNTI (SI-RNTI). If the PDCCH is for a random access response, the CRC may be masked with a random access RNTI (RA-RNTI).

FIG. 7 is a view illustrating an exemplary UL subframe structure in LTE.

Referring to FIG. 7, a UL subframe includes a plurality of (two) slots. A slot may include a different number of SC-FDMA symbols according to a CP length. The UL subframe is divided into a control region and a data region in the frequency domain. A PUSCH carrying user data such as voice is allocated to the data region. A PUCCH carrying UCI is allocated to the control region. The PUCCH includes an RB pair located at both ends of the data region along the frequency axis and hops over a slot boundary.

The PUCCH may carry the following control information.

Scheduling request (SR): information used to request UL-SCH resources. The SR is transmitted in on-off Keying (OOK).

HARQ ACK/NACK: a response signal to a DL data packet on a PDSCH. The HARQ ACK/NACK indicates whether the DL data packet has been received successfully. A 1-bit ACK/NACK is transmitted as a response to a single DL codeword and a 2-bit ACK/NACK is transmitted as a response to two DL codewords.

Channel state information (CSI): feedback information for a DL channel. The CSI includes a CQI, and multiple input multiple output (MIMO)-related feedback information includes an RI, a PMI, a precoding type indicator (PTI), and so on. The CSI occupies 20 bits per subframe.

The amount of UCI that a UE may transmit in a subframe depends on the number of SC-FDMA symbols available for transmission of the UCI. The SC-FDMA symbols available for transmission of the UCI are the remaining SC-FDMA symbols except for SC-FDMA symbols configured for transmitting RSs in the subframe. The last SC-FDMA symbol of a subframe configured to carry an SRS is additionally excluded from the SC-FDMA symbols available for transmission of the UCI. An RS is used for coherent detection of a PUCCH.

Now, a description will be given of device-to-device (D2D) communication (UE-to-UE communication).

D2D communication may or may not be network/coordination station (e.g., BS)-assisted.

Referring to FIG. 8, FIG. 8(a) illustrates a scheme in which a network/coordination station intervenes in transmission and reception of a control signal (e.g., a grant message), HARQ, CSI, and so on, and only data is transmitted and received between D2D UEs. FIG. 8(b) illustrates a scheme in which the network provides only minimal information (e.g., D2D connection information available in a corresponding cell), and D2D UEs establish a link and transmit and receive data via the link.

Based on the foregoing description, methods for efficiently configuring D2D synchronization signal (D2DSS) (transmission/reception) resources and D2DSS transmission conditions in a D2D communication environment according to the present disclosure will be described.

D2D communication refers to communication between UEs on a direct radio channel Although a UE is typically a terminal of a user, if network equipment such as an eNB transmits/receives a signal in a UE-to-UE communication scheme, the eNB may be regarded as a kind of UE to which the present disclosure is applicable. Wide area network (WAN) DL communication may refer to legacy communication such as transmission of an (enhanced) PDCCH ((E)PDCCH), a PDSCH, common reference signals (CRSs), or channel state information-reference signals (CSI-RSs) from an eNB to a UE. Or WAN communication may refer to legacy communication such as transmission of a PRACH, a PUSCH, or a PUCCH from a UE to an eNB.

While the present disclosure will be described below in the context of a 3GPP LTE system for the convenience of description, the present disclosure may be extended to other systems than the 3GPP LTE system.

Further, a UE that transmits a D2D signal is defined as a “D2D TX UE”, and a UE that receives a D2D signal is defined as a “D2D RX UE”, for the convenience of description.

Further, embodiments of the present disclosure may be extended to i) a case where some of D2D UEs participating in D2D communication are within network coverage, and the other D2D UEs are outside the network coverage (D2D discovery/communication of partial network coverage), and/or ii) a case where all of D2D UEs participating in D2D communication are within network coverage (D2D discovery/communication within network coverage), and/or iii) a case where all of D2D UEs participating in D2D communication are outside network coverage (D2D discovery/communication outside network coverage (for public safety only)).

Before a detailed description of the present disclosure, resource configuration/allocation for D2D communication will first be described.

In general, when a UE communicates with another UE on a direct radio channel, the UE may select a resource unit (RU) corresponding to specific resources from a resource pool being a set of contiguous resources, and transmits a D2D signal using the RU (i.e., a D2D TX UE operation). Then, the D2D RX UE receives information about the resource pool in which the D2D TX UE may transmit a signal, by signaling, and detects the signal of the D2D TX UE in the resource pool. Herein, i) if the D2D TX UE is within the connection coverage of an eNB, the eNB may indicate the resource pool information, and ii) if the D2D TX UE is outside the connection coverage of the eNB, another UE may indicate the resource pool information, or the resource pool may be determined to be predetermined resources.

In general, a resource pool includes a plurality of RUs, and each UE may select one or more RUs and transmit its D2D signal in the selected RUs.

FIG. 9 is an exemplary view illustrating an RU configuration for D2D communication. In the illustrated case of FIG. 9, a total of NF×NT RUs are defined by dividing total frequency resources by NF and dividing total time resources by NT. It may be said that the resource pool is repeated every NT subframes (SFs). Characteristically, one RU may occur periodically as illustrated in FIG. 9. Or to achieve time or frequency diversity, the index of a physical RU to which a logical RU is mapped may vary over time in a predetermined pattern. In this RU structure, a resource pool may mean a set of RUs available for D2D signal transmission of a D2D TX UE.

Resource pools may be categorized into a plurality of types. First, the resource pools may be classified according to the content of D2D signals. For example, the content of D2D signals may be classified as follows, and a resource pool may be configured for each D2D content type.

Scheduling assignment (SA): a signal including information about the position of resources that each D2D TX UE uses for transmission of a subsequent D2D data channel, a modulation and coding scheme (MCS) required for demodulation of the data channel, a MIMO transmission scheme for the data channel, and so on. This signal may be transmitted multiplexed with D2D data in the same RU. In this case, an SA resource pool may mean a resource pool in which an SA is transmitted multiplexed with D2D data. For the convenience of description, a predefined (or signaled) resource set used for transmission and/or reception of an SA signal is referred to as an ‘SA pool’.

D2D data channel: a pool of resources indicated by an SA, which a D2D TX UE uses for transmission of user data. If it is possible to transmit D2D data multiplexed with SA information in the same RU, only the D2D data channel except for the SA information may be transmitted in a resource pool configured for the D2D data channel. In other words, an RE used for transmission of the SA information in an individual RU of the SA resource pool is still used for transmission of D2D data in the resource pool of the D2D data channel. For the convenience of description, a predefined (or signaled) resource set used for transmission and/or reception of D2D data will be referred to as a ‘data pool’.

Discovery message: a resource pool for a message in which a D2D TX UE transmits information such as its ID or the like so that an adjacent UE may discover the D2D TX UE. For the convenience of description, for example, a predefined (or signaled) resource set used for transmission and/or reception of a discovery message will be referred to as a ‘discovery pool’.

As described before, even though D2D signals include the same type of content, different resource pools may be used according to the transmission/reception properties of the D2D signals. For example, for D2D data channels or discovery messages, different resource pools may further be defined depending on i) transmission timing determination schemes (e.g., transmission at the reception time of a reference synchronization signal, and transmission by applying a predetermined timing advance (TA) to the reception time of a reference synchronization signal) for the D2D signals, ii) resource allocation schemes (e.g., indication of transmission resources for an individual signal to an individual D2D TX UE by a cell, and autonomous selection of transmission resources for an individual signal from a pool by an individual D2D TX UE) for the D2D signals, or iii) the signal formats of the D2D signals (e.g., the number of symbols that each D2D signal occupies in one subframe or the number of subframes used for transmission of one D2D signal).

Further, resources may be allocated for transmission of a D2D data channel in the following two modes.

Mode 1: a cell directly indicates resources for use in transmission of an SA and D2D data to an individual D2D TX UE. As a result, the cell may have accurate knowledge of which UE will use which resources for D2D signal transmission. However, if the cell indicates D2D resources for transmission of every D2D signal, excessive signaling overhead may be incurred. Accordingly, the cell may allocate a plurality of SAs and/or data transmission resources by one signaling.

Mode 2: an individual D2D TX UE selects appropriate resources from a contiguous SA and data resource pool configured for a plurality of D2D TX UEs by a cell, and transmits an SA and data in the selected resources. As a result, the cell may not have accurate knowledge of which UE will use which resources for D2D signal transmission.

Resources for transmission of a discovery message may be allocated in the following two types.

TYPE 1: a discovery procedure in the case of non UE-specific resource allocation for transmission of a discovery signal. Herein, resources may be for all UEs or a group of UEs.

TYPE 2: a discovery procedure in the case of UE-specific resource allocation for transmission of a discovery signal.

TYPE 2A: resources are allocated at a specific transmission instance of each discovery signal.

TYPE 2B: resources are allocated semi-persistently for transmission of a discovery signal.

FIG. 10 illustrates periodic occurrences of a discovery message-related resource pool (i.e., ‘discovery resource pool’). In FIG. 10, the occurrence period of the resource pool is written as a ‘discovery resource pool period’. In FIG. 10, among a plurality of discovery resource pools configured within one discovery resource pool period, a specific discovery resource pool(s) may be defined as a serving cell-related discovery TX/RX resource pool(s), and the other discovery resource pool(s) may be defined as a neighbor cell-related discovery RX resource pool(s).

A description will be given of methods for configuring D2DSS resources, and D2DSS transmission conditions, as proposed by the present disclosure based on the above description.

First, an in-coverage (or in-network (in-NW)) UE will be described.

A maximum of one D2DSS resource per cell may be configured for in-coverage UEs. The D2DSS resource includes a periodic SF satisfying the following Condition i) and Condition ii). A D2DSS may be transmitted in the periodic SF (e.g., an eNB will utilize resources unused for D2DSS transmission (for WAN communication)). i) D2DSS resource periods are equal for in-coverage and out-of-coverage, and may be preset to 40 ms. ii) When the D2DSS resource is configured, a time offset may be set in units of an SF, and D2DSS resource offsets of neighbor cells may be signaled by an SIB (the D2DSS resource offsets are time offsets in units of an SF with respect to SFN #0 of a serving cell).

A UE transmitting an SA or D2D data transmits a D2DSS in each SF satisfying (all or a part of) the following conditions within the D2DSS resource.

The SF does not collide with a cellular transmission, from the viewpoint of the UE.

A predefined condition such as UE capabilities is satisfied.

The SF is within an SA or a data period during which the SA or data is transmitted.

The UE is in RRC Connected and an eNB indicates initiation of D2DSS transmission (by dedicated signaling), and/or when the UE does not transmit an SA or D2D data in an SF within the SA or a data period, a predefined other condition is satisfied, and/or all (or a part) of the following conditions are satisfied.

A reference signal received power (RSRP) threshold is set for D2DSS transmission related to D2D communication, and the RSRP threshold is configured by an SIB. For example, the threshold may be set to one of {−∞, −115, . . . , −60 (incremented by 5 each time), +∞} dBm.

The RSRP of the UE is lower than the threshold.

The eNB does not indicate discontinuation of the D2DSS transmission (by dedicated signaling).

For each discovery pool, in the case where the first SF of the discovery pool is a D2DSS resource, if (all or a part of) the following conditions are satisfied, a discovery UE transmits a D2DSS in the SF. Otherwise, if a D2DSS resource closest to the discovery pool, before the starting time of the discovery pool satisfies (all or a part of) the following conditions, the discovery UE transmits a D2DSS in the SF.

The SF does not collide with a cellular transmission from the view point of the UE.

The UE does not scan for another D2DSS.

A predefined condition such as UE capabilities is satisfied.

The UE transmits a discovery message in the discovery pool.

The UE is RRC Connected and the eNB indicates initiation of D2DSS transmission (by dedicated signaling), and/or all (or a part) of the following conditions are satisfied.

An RSRP threshold is set for D2DSS transmission related to D2D communication, and the RSRP threshold is configured by an SIB. For example, the threshold may be set to one of {−∞, −115, . . . , −60 (incremented by 5 each time), +∞} dBm.

The RSRP of the UE is lower than the threshold.

The eNB does not indicate discontinuation of the D2DSS transmission (by dedicated signaling).

Further, an out-of-coverage UE will be described below. The out-of-coverage UE does not transmit a D2DSS in more D2DSS resources than one. For example, two D2DSS resources are used for out-of-coverage. For example, the positions of D2DSS resources (with respect to DFN#0) may be preset or signaled.

If a D2D RX UE receives neighbor cell-related synchronization error information of w1/w2 (by predefined higher-layer signaling), the D2D RX UE assumes a discovery reference synchronization window of size ±w1/±w2 for a neighbor cell D2D resource (and/or a neighbor cell discovery resource pool) (refer to [Table 3]).

TABLE 3 If higher layer indicates w1 in a given neighbor cell, UE may assume for the purpose of discovery a reference synchronization window of size +/−w1 ms for that neighbour cell with respect to neighbour cell D2DSS resource w1 is a fixed value and decided UE may assume D2DSS is transmitted in that cell If higher layer indicates w2 in a given neighbor cell, UE may assume for the purpose of discovery a reference synchronization window of size +/−w2 ms for that neighbour cell with respect to neighbour cell discovery resource Exact value of w2 is decided RAN1 recommend w2 as not greater than CP length (of the order of CP length) UE expects that D2DSS indicated by the resource pool configuration appears only within signaled reference synchronization window

FIG. 11 is a view referred to for describing configuration of a D2DSS SF and a D2DSS relay SF for the above-described in-coverage UE and out-of-coverage UE.

Referring to FIG. 11, a maximum of one D2DSS resource (e.g., D2DSS SF) per cell may be configured for an in-coverage UE (e.g., UEa) located within the coverage of an eNB. On the other hand, for an out-of-coverage UE located outside the coverage of the eNB, a (another) D2DSS resource (e.g., a D2DSS relay SF) may be configured along with a (one) D2DSS resource aligned with the D2DSS resource for the in-coverage UE.

FIG. 12 illustrates the positions of resource pools in which a D2DSS is transmitted. Referring to FIG. 12, a D2DSS may be transmitted in the first SF of a discovery pool ((a)) or in an SF closest to the discovery pool, before the starting time of the discovery pool ((b)).

Different conditions for D2DSS transmission may be set for an in-coverage UE and an out-of-coverage UE. For example, it may be indicated to the in-coverage UE whether a D2DSS will be transmitted by i) dedicated signaling from an eNB, or ii) a (preset or signaled) RSRP reference. For example, it may be determined for an out-of-coverage UE whether a D2DSS will be transmitted, based on (energy) measurement/detection in demodulation RSs (DMRSs) of a physical sidelink broadcast channel (PSBCH). For example, if no signal (e.g., no PSBCH DMRS) having a strength equal to or greater than a predetermined threshold is measured/detected (within a predetermined area/distance), the out-of-coverage UE determines that there is no synchronization source (within the predetermined area/distance), and transmits a D2DSS (as an independent synchronization source (ISS)). While discovery (pool)-related D2DSS transmission has been focused in FIG. 12, for the convenience of description, the present disclosure may also be extended to D2D communication (e.g., SA or D2D data) (pool)-related D2DSS transmission.

A description will first be given of an operation of an in-NW UE based on the foregoing description. Characteristically, D2DSS transmission may be optional to D2D-capable UEs. Accordingly, for example, only a D2DSS-capale UE preferably transmits a D2DSS.

A discovery UE transmits a D2DSS in a single SF during each discovery period. This operation may be sufficient for discovery performed only for an in-NW UE. In other words, as the in-NW UE is synchronized with a cell, a frequency error between a TX UE and an RX UE is limited, and D2DSS detection in a single SF is highly reliable. In this case, an additional condition may not be needed for D2DSS scanning because a serving cell provides D2DSS resources of neighbor cells and the D2DSS resources of a plurality of cells may be separated in time according to a network configuration. Further, a UE may not transmit a discovery signal in a resource pool, one of the reasons for which is collision with a WAN UL TX.

Therefore, one of the foregoing discovery-related D2DSS transmission conditions, “the UE transmits a discovery message in the discovery pool” needs to be changed to “the UE intends to transmit a discovery message in the discovery pool”.

In regards to communication, it may first be determined whether D2DSS transmission needs to precede SA transmission (herein, data cannot be transmitted before the SA transmission). This is because there may not be a D2DSS resource before an SA SF within an SA/data period. In this case, a D2DSS may be transmitted after transmission of the SA. That is, if synchronization is required before SA reception, conditions similar to the foregoing conditions for discovery (discovery-related D2DSS transmission) may be additionally set.

In this case, however, D2DSS transmission in a single SF may not provide reliable synchronization performance to an out-NW UE(s) that may have a large initialization frequency offset. Accordingly, it is preferred that a D2DSS is transmitted in a plurality of SFs before SA transmission. For example, time limitation may be needed for the preceding D2DSS transmission. This is because if a time gap between a D2DSS SF and an SA SF is large, a UE has difficulty in accurately predicting the intention of SA transmission.

Further, in the case where an SA or data is not transmitted within an SA/data period, whether to transmit a D2DSS will be described. Because out-NW UEs need to receive a D2DSS for communication, an operation for discovery and an operation for communication need to be different. Specifically, because out-NW UEs may have a large frequency error, D2DSS detection should be highly reliable.

For fast synchronization of out-NW UEs, in-NW UEs need to transmit D2DSSs consecutively during a (preset) minimum time period. Thus, the out-NW UEs may detect at least one D2DSS in a set of contiguous D2DSS transmission SFs.

Further, because the out-NW UEs need to select a synchronization reference, measure a D2DSS to determine whether a D2DSS transmission condition is satisfied, and average proper (or reliable) measurements across a plurality of D2DSS SFs, random on-off of D2DSS transmission in units of 40 ms is preferably avoided.

For this purpose, if a preset specific condition is satisfied, a UE may be configured to transmit a D2DSS even though an SA or D2D data is not transmitted within an SA/data period. Hereinbelow, this will be referred to as a “condition for continuing D2DSS transmission”.

The “condition for continuing D2DSS transmission” may be based on the principle that if a UE has transmitted a D2DSS at a previous time instant, the UE continues the D2DSS transmission during a (preset) time period. This principle may ensure continuous D2DSS transmission helpful for D2DSS detection and measurement of out-NW UEs.

Therefore, the following Option 1-1 to Option 1-3 may be considered in the present disclosure. FIG. 13 is a view referred to for describing Option 1-1 to Option 1-3. Option 1-1 to Option 1-3 will be described with reference to FIG. 13.

Option 1-1: a “D2DSS transmission timer” may be defined. If a UE transmits a D2DSS in SF #n under the condition that “the SF is within the SA or D2D data period in which the SA or data is transmitted”, the UE may transmit the D2DSS continuously in SF #n+40, SF #n+80, . . . SF #n+K*40 in spite of the absence of an SA/data to be transmitted. Herein, K corresponds to the “D2DSS transmission timer”.

Option 1-2: a total DFN range may be divided into a plurality of time partitions. On the assumption that the DFN range is from 0 to 1023 (i.e., one D2D frame is 10 ms long), DFN partition x includes D2D frames x, x+1, . . . , x+M−1 (i.e., in the case of division into 1024/M DFN partitions). If a UE transmits a D2DSS in an SF included in DFN partition X, the UE continues the D2DSS transmission in the remaining D2DSS SFs of DFN partition X. This option is advantageous in that after an RX UE decodes a DFN on a linked (or associated) PD2DSCH, the RX UE may determine a potential D2DSS transmission change-related time instance.

Option 1-3: a “D2DSS measurement period” may be defined, and a UE that has transmitted a D2DSS in a specific SF transmits the D2DSS during a D2DSS measurement period linked to the SF. For example, it may be defined that a closest D2DSS measurement period (to the specific SF carrying the D2DSS of the UE) is linked (to the specific SF).

In relation to the above conditions, it is necessary to make sure that a UE does not transmit a D2DSS in an SF that does not satisfy the D2DSS transmission conditions. The eNB may determine at least a subset of SFs in which a D2DSS is not transmitted, and use the D2DSS resources of these SFs for cellular transmission (communication).

That is, in the case of in-coverage UEs,

A UE transmitting an SA or D2D data transmits a D2DSS in each SF satisfying (all or a part of) the following conditions within a D2DSS resource.

The SF does not collide with a cellular transmission, from the viewpoint of the UE.

A D2DSS-capable UE.

The SF is within the SA or D2D data period in which the SA or data is transmitted, and/or the SF is within Xms from an SF in which the UE intends to transmit the SA, and/or the SF satisfies a “condition for continuing D2DSS transmission”.

The UE is in RRC Connected state and the eNB indicates initiation of D2DSS transmission (by dedicated signaling), and/or all (or a part) of the following conditions are satisfied.

An RSRP threshold for D2D communication-related D2DSS transmission is set, and the threshold is set by an SIB. For example, the threshold may be set to one of {−∞, −115 . . . −60 (incremented by 5 each time), +∞} dBm.

The RSRP value of the UE is smaller than the threshold.

The eNB does not indicate discontinuation of the D2DSS transmission (by dedicated signaling).

In regards to a discovery UE, for each discovery pool, if the first SF of the discovery pool is a D2DSS resource and (all or a part of) the following conditions are satisfied, the discovery UE transmits a D2DSS in the SF. Otherwise, if a D2DSS resource closest to the discovery pool before the starting time of the discovery pool satisfies (all or a part of) the following conditions, the UE transmits a D2DSS in the SF.

The SF does not collide with a cellular transmission, from the viewpoint of the UE.

A D2DSS-capable UE.

The UE intends to transmit a discovery message in the discovery pool.

The UE is in RRC Connected state and the eNB indicates initiation of D2DSS transmission (by dedicated signaling), and/or all (or a part) of the following conditions are satisfied.

An RSRP threshold for D2D discovery-related D2DSS transmission is set, and the threshold is set by an SIB. For example, the threshold may be set to one of {−∞, −115 . . . −60 (incremented by 5 each time), +∞} dBm.

The RSRP value of the UE is smaller than the threshold.

The eNB does not indicate discontinuation of the D2DSS transmission (by dedicated signaling).

If the above conditions are not satisfied, the UE does not transmit a D2DSS.

Further, for the “condition for continuing D2DSS transmission”, the following three options, Option 2-1 to Option 2-3 may be considered.

Option 2-1: a D2DSS timer may be defined, and a UE that has transmitted a D2DSS according to the SA/data transmission condition may continue the D2DSS transmission without SA/data transmission until the timer expires.

Option 2-2: a total DFN range is divided into a plurality of DFN partitions, and a UE that has transmitted a D2DSS in an SF continues the D2DSS transmission during a DFN partition.

Option 2-3: a D2DSS measurement period is defined, and a UE that has transmitted a D2DSS in an SF continues the D2DSS transmission during a linked D2DSS measurement period.

Further, for D2DSS reception, a reference synchronization window for discovery may also be applied to communication, because the same D2DSS resource is shared between discovery and communication. After receiving a discovery resource pool, a UE may determine an accurate position of D2DSS transmission for discovery. Further, in the case of w2, a D2DSS may be omitted or transmitted outside a synchronization window. In consideration of this, a D2DSS (reception)-related UE assumption within the synchronization window may be limited to w1.

Accordingly, the reference synchronization window may be applied to both discovery and communication based on the principle that the “UE expects that a D2DSS indicated by the resource pool configuration appears only within a signaled reference synchronization window if w1 is indicated”.

Now, out-NW UEs will be described. For example, it is important to minimize the number of D2DSSs that an out-NW UE needs to track. That is, since the UE can track only a limited number of D2DSSs, if the number of D2DSSs related to an incoming SA and data exceeds a limit, the UE may not receive all incoming SAs and data.

Therefore, since a UE capability of tracking different timings is limited, the following UE operations need to be considered. That is,

-   1) a UE synchronized with a D2DSS transmits the same D2DSS to form a     synchronization cluster sharing a common timing. -   2) only a data TX UE may become an ISS. -   3) if a UE has transmitted a specific sequence during a previous     period, the UE excludes the same D2DSS sequence when reselecting a     D2DSS.

Therefore, a D2DSS sequence selection procedure for out-NW UEs is performed in the following three steps. For the convenience of description, for example, “a set of D2DSS sequence(s) transmitted by a UE when a transmission timing reference is an eNB” is referred to as D2DSS_net, and “a set of D2DSS sequence(s) transmitted by a UE when a transmission timing reference is not an eNB” is referred to as D2DSSue_oon.

Step 1: if an out-NW UE selects D2DSS X of D2DSSue_net as its TX timing reference, the UE selects D2DSS Y from D2DSSue oon and transmits the selected D2DSS Y. The UE may make the selection randomly, or may avoid/prevent selection of a D2DSS detected during a TX timing reference selection procedure.

Step 2: if the UE selects D2DSS Z from D2DSSue_oon as its TX timing reference, the UE transmits the same D2DSS Z, when transmitting a D2DSS.

Step 3: if the UE has D2D data traffic to be transmitted, the UE may become an ISS using a D2DSS selected randomly from D2DSSue_oon.

Step 2 enables a D2DSS relay operation that reduces the number of D2DSSs in the system, in consideration that a UE synchronized with a D2DSS transmits the same D2DSS to form a synchronization cluster sharing a common timing.

Further, if the ISS has transmitted a specific sequence during a previous period, the ISS that has transmitted (or started to transmit) D2DSS Z assumes that D2DSS Z has not been detected, for synchronization with another D2DSS, in consideration that the same D2DSS sequence is excluded in D2DSS reselection. In other words, before reselection, the ISS may maintain the ISS operation only when a D2DSS other than a D2DSS transmitted by the ISS is not detected during reselection. After this operation, the out-NW UE may determine a D2DSS sequence for use in D2DSS transmission.

Further, a “detecting D2DSS” will be described in detail in the present disclosure. If an associated PD2DSCH is not decoded accurately or PD2DSCH reception quality is very poor, it is not proper to consider that a D2DSS has been detected and thus use the D2DSS as a reliable synchronization source. Specifically, if the associated PD2DSCH reception quality (e.g., the RSRQ of a PD2DSCH DM RS) is lower than a specific level, a UE may assume that a D2DSS has not been detected (therefore, the D2DSS does not affect D2D synchronization of the UE).

Accordingly, the following configurations may be applied for selection of a D2DSS sequence according to the present disclosure.

-   If a UE selects D2DSSue oon as its TX timing reference, the UE     transmits the same D2DSS. -   The UE assumes that UEs transmitting the same D2DSS have been     synchronized.

A condition that the out-NW UE transmits a D2DSS using a D2DSS sequence selected in the above operation will be described additionally. Basically, the D2DSS transmission condition formulation for an in-NW UE may be reused. If a UE other than an ISS detects a D2DSS from another UE, the UE transmits a D2DSS irrespective of whether its SA/data will be transmitted. That is, an additional condition may be required for D2DSS transmission of a non-ISS UE. For example, an RSRP threshold may be replaced with a D2DSS measurement threshold, and eNB configurations may be removed.

For reliable D2DSS detection and measurement of an out-NW UE, the afore-described D2DSS transmission preceding SA transmission and the afore-described condition for continuing D2DSS transmission may still be needed.

Therefore, according to the present disclosure, the following conditions may be configured to determine whether an out-NW UE will transmit a D2DSS in a single SF.

In regards to an out-of-coverage UE,

in the case where the UE is an independent synchronization source (i.e., ISS), if each SF in a D2DSS resource that the UE has selected for D2DSS transmission is i) an SF within an SA or D2D data period in which the SA or D2D data is transmitted, and/or ii) an SF within X ms from an SF in which the UE intends to transmit the SA, and/or iii) an SF satisfying the “condition for continuing D2DSS transmission”, the UE should transmit a D2DSS in the SF.

in the case where the UE is not an ISS, i) if each SF of a D2DSS resource that the UE does not use in receiving its TX synchronization reference is an SF within an SA or D2D data period in which the SA or D2D data is transmitted, or(/and) an SF within X ms from an SF in which the UE intends to transmit the SA, or(/and) an SF satisfying the “condition for continuing D2DSS transmission”, or(/and) if a D2DSS of the TX synchronization reference is detected within a (preset) time window, or(/and) ii) if the D2DSS measurement of the TX timing reference is lower than a threshold, the UE should transmit a D2DSS.

Further, only two D2DSS resources are configured as D2DSS transmission resources, and an out-NW UE receives a D2DSS in one D2DSS resource from its synchronization reference and transmits a D2DSS in the other D2DSS resource.

Further, an out-of-coverage UE uses a periodic synchronization resource in D2DSS transmission. For example, a PD2DSCH may be transmitted (when supported), for D2DSS transmission. For example, the size of the synchronization resource may be predefined and the period of the synchronization resource may also be preset.

When a D2DSS synchronization source transmits a D2DSS in a synchronization resource, it transmits the D2DSS in at least one synchronization resource, and receives a D2DSS at least in the other synchronization resource(s). The synchronization resource that transmits (and/or) receives a D2DSS may be predetermined. In an additional example, a timing offset may be set between the synchronization resource for D2DSS reception and the synchronization resource for D2DSS transmission.

Therefore, according to the present disclosure, the UE should not transmit any (other) D2D signal/channel in a (D2DSS) SF unused for its D2DSS transmission, to thereby make sure to receive D2DSSs from other UEs.

Further, whether a D2D-silent period is required during a D2DSS reselection procedure of a UE will be described. Even though a synchronization resource occurs periodically and a UE does not transmit any (other) D2D signal/channel in (other) synchronization resources except for synchronization resources used for its D2DSS transmission, eNBs and UEs that are not synchronized with the periodic synchronization resources may transmit D2DSSs (in synchronization resources unused for transmission of the UE's D2DSS). Accordingly, it is necessary to define a “D2D-silent period” for D2D scanning without disturbance (or interference) from transmissions of nearby D2D UEs, so that UEs may efficiently scan potential asynchronous D2DSSs. If this period is not defined, the out-NW UE may not detect a weak D2DSS with high priority transmitted by an eNB or an in-NW UE, due to interference from other out-NW UEs.

Therefore, the present disclosure may support scanning of other synchronization sources by an out-NW UE by defining a “D2D-silent period” as a multiple of the length of a D2DSS period.

A description will be given of a WAN DL signal reception operation that a D2D RX UE with a single RX chain (i.e., “SRXCH_D2D RX UE”) assumes when receiving a D2D discovery signal.

TABLE 4 For FDD carriers: At least for UEs with a single Rx chain (FFS subject to the UE capability discussion whether this also applies for UEs with a shared D2D/cellular Rx chain), a UE that is receiving D2D discovery signals on an UL carrier is not expected to read DL signals on the DL carrier paired to such UL carrier during the subframes belonging to the D2D discovery pools on that UL carrier as well as one subframe preceding and following these subframes The discovery pools are configured by the eNB by broadcast or  UE-specific signaling FFS: For RRC_CONNECTED UEs, 1 bit may be signalled  using RRC signaling indicating whether this rule  applies or not (on a per UE basis) Cellular measurement gaps subframes are excluded from this rule Paging reception is prioritized over D2D reception For TDD carriers: A UE configured by the eNB to monitor D2D on a certain carrier is expected to read DL signals on that carrier according to legacy procedures.

[Table 3] illustrates an exemplary synchronization assumption/configuration with which a D2D RX UE receives an inter-cell discovery signal (or a neighbor cell discovery signal). For example, if a D2D RX UE receives neighbor cell-related synchronization error information of w1/w2 (by predefined higher-layer signaling), the D2D RX UE assumes a discovery reference synchronization window of size ±w1/±w2 for a neighbor cell D2D resource (and/or a neighbor cell discovery resource cell) (refer to [Table 3]).

In a specific example, if a neighbor-cell D2DSS resource is configured in serving-cell SF#N, the D2D RX UE assumes that it may receive a neighbor-cell D2DSS within a range from ‘SF#N−w1’ to ‘SF#N+w1’. If the D2D RX UE receives neighbor cell-related synchronization error information of w2 (by predefined higher-layer signaling), the D2D RX UE assumes a discovery reference synchronization window of size ±w2 for a neighbor-cell discovery resource. In a specific example, if a neighbor-cell discovery resource is configured in serving-cell SF#K, the D2D RX UE assumes that it may receive a neighbor-cell discovery signal within a range from ‘SF#K−w2’ to ‘SF#K+w2’.

Now, the present disclosure proposes methods for efficiently receiving a WAN DL signal(s) in a DL SF(s) overlapped at least partially in the time domain, when a D2D RX UE with a single RX chain receives a D2D signal(s) in a preset or signaled D2D signal resource pool and/or a D2DSS(s) (linked to the D2D signal resource pool) under a D2D communication environment.

For the convenience of description, a D2D RX UE with a single RX chain will be referred to as an “SRXCH_D2D RX UE”. For example, the term “SRXCH_D2D RX UE” may be extended to (interpreted as) a UE having fewer RX chains than the number of RX chains needed for simultaneous reception of a WAN DL signal/channel and/or a D2D signal/channel. For example, in view of the single RX chain, the SRXCH_D2D RX UE has difficulty in simultaneously receiving a D2D signal(s) (i.e., UL carrier #X) and a WAN DL signal(s) (i.e., DL carrier #X paired with UL carrier #X), or a D2DSS(s) and a WAN DL signal(s), which are overlapped at least partially (i.e., fully or partially) on different carriers (or in different frequency bands). For example, the SRXCH_D2D RX UE receives i) a D2D signal(s) (/D2DSS(s)) and a WAN DL signal(s) or ii) a D2DSS(s) and a WAN DL signal(s), which are transmitted in different time areas of different carriers (or frequency bands), by carrier (or frequency band) switching of the single RX chain.

A DL SF(s) overlapped at least partially (i.e., partially or fully) in the time domain may be interpreted as at least one of i) a DL SF(s) overlapped in the time domain at least partially (i.e., partially or fully) with all SF(s) within a time period to which a D2D signal resource pool configuration-related bitmap is applied, ii) a DL SF(s) overlapped in the time domain at least partially (i.e., partially or fully) with a (D2D signal resource pool or D2D signal(s) reception-related valid) D2DSS(s), iii) a DL SF(s) overlapped in the time domain at least partially (i.e., partially or fully) with one SF preceding and following a D2D signal resource pool (illustrated in [Table 4]) (i.e., an SF(s) for ensuring a time required for carrier (or frequency band) switching of a single RX chain), iv) a DL SF(s) overlapped in the time domain at least partially (i.e., partially or fully) with an SF(s) configured actually as a D2D SF(s) among a SF(s) within a time period to which a D2D signal resource pool configuration-related bitmap is applied, or v) a DL SF(s) overlapped in the time domain at least partially (i.e., partially or fully) with one SF preceding and following a (D2D signal resource pool or D2D signal(s) reception-related valid) D2DSS(s) (i.e., an SF(s) for ensuring a time required for carrier (or frequency band) switching of a single RX chain).

For the convenience of description, such a DL SF(S) will be referred to as an “INV_DL SF(S)” (or “DL gap”), and when a SRXCH_D2D RX UE receives a D2D signal(s)/D2DSS(S), it may be interpreted that the SRXCH_D2D RX UE does not receive a WAN DL signal(s) in a corresponding INV_DL SF(S) (or DL GAP). Further, a preset or signaled D2D signal resource pool may be interpreted as at least one of a serving cell-related D2D signal resource pool and/or a neighbor cell-related D2D signal resource pool. In relation to i) D2D SA reception, ii) D2D data reception, iii) D2D discovery signal reception, or iv) a D2D discovery pool, at least one valid D2DSS resource position may be assumed according to the afore-described D2DSS resource configuration.

The following embodiments of the present disclosure are based on the assumption that an SRXCH_D2D RX UE receives a discovery signal(s) in a preset or signaled (serving-cell/neighbor-cell) D2D signal resource pool, and/or a D2DSS(s) (linked to the (serving-cell/neighbor-cell) D2D signal resource pool). However, the proposed methods of the present disclosure may be extended to a case in which a different type of D2D signal (e.g., a D2D communication signal) is received. It may also be configured that the proposed methods are applied restrictively to FDD carrier-based D2D signal/D2DSS reception.

<Method 1>

According to the present disclosure, when an SRXCH_D2D RX UE receives a D2DSS(s) linked to a (serving-cell/neighbor-cell) D2D signal resource pool, the SRXCH_D2D RX UE may be configured to assume that not only i) a DL SF(s) overlapped in the time domain at least partially (i.e., partially or fully) with a corresponding D2DSS resource(s) (or a D2DSS SF(s)) but also ii) a DL SF(s) overlapped in the time domain at least partially (i.e., partially or fully) with one SF preceding and following a D2DSS resource(s) (or D2DSS SF(S)) (i.e., an SF(s) for ensuing a time required for carrier (or frequency band) switching of a single RX chain) is an INV_DL SF(S).

Further, when the SRXCH_D2D RX UE receives a discovery signal(s) in a preset or signaled neighbor-cell discovery resource pool and/or a D2DSS(s) (linked to the neighbor-cell discovery resource pool), an INV_DL SF(S) may be defined/configured according to at least a part (i.e., a part or all) of the following rules.

The following description is based on the assumption that the SRXCH_D2D RX UE receives neighbor cell-related synchronization error information of w1 (or w2) (by predefined higher-layer signaling) (refer to [Table 4]). In this case, i) the SRXCH_D2D RX UE assumes that a neighbor-cell D2DSS may be received within a range from ‘SF#N−w1’ to ‘SF#N+w1’ (i.e., in the case where a neighbor-cell D2DSS resource is configured in serving-cell SF#N), or ii) the SRXCH_D2D RX UE assumes that a neighbor-cell discovery signal may be received within a range from ‘SF#K−w2’ to ‘SF#K+w2’ (i.e., in the case where a neighbor-cell discovery resource is configured in serving-cell SF#K).

EXAMPLE 1-1

According to Embodiment 1 of the present disclosure, the SRXCH_D2D RX UE should blind-search an area ranging from ‘SF#N−w1’ to ‘SF#N+w1’, for neighbor-cell D2DSS reception (or detection) (i.e., in the case where a neighbor-cell D2DSS resource is configured in serving-cell SF#N).

For this operation, when the SRXCH_D2D RX UE receives a neighbor-cell D2DSS (linked to a preset or signaled neighbor-cell discover resource pool), the SRXCH_D2D RX UE may be configured to assume that not only i) a DL SF(s) overlapped in the time domain at least partially (i.e., partially or fully) with an area ranging from ‘SF#N−CEILING(w1)’ to ‘SF#N+CEILING(w1)’, but also ii) a DL SF(s) overlapped in the time domain at least partially (i.e., partially or fully) with one SF preceding or following the area ranging from ‘SF#N−CEILING(w1)’ to ‘SF#N+CEILING(w1)’ (i.e., an SF(s) for ensuring a time required for carrier (or frequency band) switching of a single Rx chain) is an INV_DL SF(S).

In other words, the SRXCH_D2D RX UE assumes that a DL SF(s) overlapped in the time domain at least partially (i.e., partially or fully) with an area ranging from ‘SF#N−CEILING(w1)−1’ to ‘SF#N+CEILING(w1)+1’ is an INV_DL SF(S). Herein, CEILING(X) represents a function of deriving a smallest integer equal to or greater than X.

In another example, in the same situation (in the case where w1 is smaller than a preset or signaled threshold, that is, a time required for carrier (or frequency band) switching is secured without configuring an additional INV_DL SF(S)), the SRXCH_D2D RX UE may be configured to finally assume that a DL SF(s) overlapped in the time domain at least partially (i.e., partially or fully) with an area ranging from ‘SF#N−CEILING(w1)’ to ‘SF#N+CEILING(w1)’ is an INV_DL SF(s).

In another example, the SRXCH_D2D RX UE should blind-search an area ranging from ‘SF#P−w1’ to ‘SF#P+w1’, for neighbor-cell discovery signal reception(/detection) (i.e., in the case where a neighbor-cell discovery resource is configured in serving-cell SF#P) due to a D2D RX neighbor cell-related synchronization error (in the same situation).

For this operation, the SRXCH_D2D RX UE may be configured to assume that i) a DL SF(s) overlapped in the time domain at least partially (i.e., partially or fully) with an area ranging from ‘(STARTING) SF−CEILING(w1)−1 of a neighbor-cell discovery pool’ to ‘(ENDING) SF+CEILING(w1)+1 of the neighbor-cell discovery pool’ is an INV_DL_SF(s), or ii) a DL SF(s) overlapped in the time domain at least partially (i.e., partially or fully) with an area ranging from ‘(STARTING) SF−CEILING(w1) of a neighbor-cell discovery pool’ to ‘(ENDING) SF+CEILING(w1) of the neighbor-cell discovery pool’ is an INV_DL_SF(s) (in the case where w1 is smaller than a preset or signaled threshold), or iii) a DL SF(s) overlapped in the time domain at least partially (i.e., partially or fully) with an area ranging from ‘SF#P−CEILING(w1)−1’ to ‘SF#P+CEILING(w1)+1’ (or (in the case where w1 is smaller than a preset or signaled threshold) an area ranging from ‘SF#P−CEILING(w1)’ to ‘SF#P+CEILING(w1)+1’) is an INV_DL_SF(s) (i.e., it may be interpreted that when an INV_DL SF(s) is configured, only an SF(s) actually configured as a discovery SF(s) from among an SF(s) within a time period to which a neighbor-cell discovery pool configuration-related bitmap is applied is considered).

In another example, if the ending SF of a time period to which a (serving-cell/neighbor-cell) discovery pool configuration-related bitmap is applied is a non-D2D SF (or a non-discovery SF), it may be configured that a DL SF(s) overlapped in the time domain at least partially (i.e., partially or fully) with one SF following the ending SF is not assumed to be an INV_DL SF(S).

In another example, if SFs are arranged in the order of “non-D2D SF, D2D SF, non-D2D SF” in the time period to which the (serving-cell/neighbor-cell) discovery pool configuration-related bitmap is applied, it may be configured that a DL SF(s) overlapped in the time domain at least partially (i.e., partially or fully) with one SF preceding and following the D2D SF is assumed to be an INV_DL SF(S). Or if SFs are arranged in the order of “non-discovery SF, discovery SF, non-discovery SF” in the time period to which the (serving-cell/neighbor-cell) discovery pool configuration-related bitmap is applied, it may be configured that a DL SF(s) overlapped in the time domain at least partially (i.e., partially or fully) with one SF preceding and following the discovery SF is assumed to be an INV_DL SF(S).

In another example, if in the time period to which the (serving-cell/neighbor-cell) discovery pool configuration-related bitmap is applied, D2DSS transmission is configured in a (UL) SF(s) for which the bitmap does not indicate ‘1’ (i.e., meaning that the (UL) SF(s) is configured as a D2D SF (or a discovery SF)) or the (UL) SF(s) is defined as a D2DSS resource, it may be configured that D2D transmission is performed exceptionally in the (UL) SF(s). On the contrary, if in the time period to which the (serving-cell/neighbor-cell) discovery pool configuration-related bitmap is applied, D2DSS transmission is configured in a (UL) SF(s) for which the bitmap does not indicate ‘1’ (i.e., meaning that the (UL) SF(s) is configured as a D2D SF (or a discovery SF)) or the (UL) SF(s) is defined as a D2DSS resource, it may be configured that D2D transmission is not performed exceptionally in the (UL) SF(s).

In another example, if the SRXCH_D2D RX UE blind-searches an area ranging from ‘SF#N−w2’ to ‘SF#N+w2’, for reception/detection of a neighbor-cell D2DSS (linked to a preset or signaled neighbor-cell discovery resource pool) (i.e., in the case where a neighbor-cell D2DSS resource is configured in serving-cell SF#N), the SRXCH_D2D RX UE may be configured to assume that a DL SF(s) overlapped in the time domain at least partially (i.e., partially or fully) with one of an area ranging from ‘SF#N−1’ to ‘SF#N+1’, an area ranging from ‘SF#N−CEILING(w2)−1’ to ‘SF#N+CEILING(w2)+1’, and an area ranging from ‘SF#N−FLOOR(w2)−1’ to ‘SF#N+FLOOR(w2)+1’ is an INV_DL SF(S).

In another example, (in the same situation) the SRXCH_D2D RX UE should blind-search an area ranging from ‘SF#P−w2’ to ‘SF#P+w2’, for reception/detection of a neighbor-cell discovery signal due to a D2D RX neighbor cell-related synchronization error (i.e., in the case where a neighbor-cell discovery resource is configured in serving-cell SF#P).

For this operation, the SRXCH_D2D RX UE may be configured to assume that a DL SF(s) overlapped in the time domain at least partially (i.e., partially or fully) with i) an area ranging from ‘the starting SF−1 of a neighbor-cell discovery pool’ to ‘the ending SF+1 of the neighbor-cell discovery pool’, ii) an area ranging from ‘the starting SF−CEILING(w2)−1 of the neighbor-cell discovery pool’ to ‘the ending SF+CEILING(w2)+1 of the neighbor-cell discovery pool’, and iii) an area ranging from ‘the starting SF−FLOOR(w2)−1 of the neighbor-cell discovery pool’ to ‘the ending SF+FLOOR(w2)+1 of the neighbor-cell discovery pool’ is an INV_DL SF(S).

In another example, as the rule described in [Table 4], “paging reception (and/or SIB reception) is prioritized over D2D reception” is applied, if an (RRC_IDLE) D2D UE should receive a paging signal (and/or an SIB) (SF#N) during at least one of i) reception of a (neighbor-cell/serving-cell) discovery signal in a (neighbor-cell/serving-cell) D2D signal resource pool and ii) reception of a (neighbor-cell/serving-cell) D2DSS linked to the (neighbor-cell/serving-cell cell) D2D signal resource pool, the (RRC_IDLE) D2D UE may be configured not to receive a discovery signal in an SF(s) of the (neighbor-cell/serving-cell cell) D2D signal resource pool, overlapped in the time domain at least partially (i.e., partially or fully) with an area ranging from ‘SF#N−1’ to ‘SF#N+1’ (or ‘SF#N’).

In another example, as the rule described in [Table 4], “paging reception (and/or SIB reception) is prioritized over D2D reception” is applied, if the (RRC IDLE) D2D UE should receive a paging signal (and/or an SIB) (SF#N) during at least one of i) reception of a (neighbor-cell/serving-cell) discovery signal in a (neighbor-cell/serving-cell) D2D signal resource pool and ii) reception of a (neighbor-cell/serving-cell) D2DSS linked to the (neighbor-cell/serving-cell cell) D2D signal resource pool, the (RRC IDLE) D2D UE may be configured not to receive a D2DSS in a (neighbor-cell/serving-cell) D2DSS(s) (or D2DSS resource) overlapped in the time domain at least partially (i.e., partially or fully) with an area ranging from ‘SF#N−1’ to ‘SF#N+1’ (or ‘SF#N’).

In another example, the (RRC_IDLE) D2D UE may be configured not to receive a discovery signal in a (neighbor-cell/serving-cell) D2D signal resource pool overlapped in the time domain at least partially (i.e., partially or fully) with a reception time of the paging signal (and/or SIB).

EXAMPLE 1-2

According to Embodiment 1 of the present disclosure, if i) the difference between a preset or signaled neighbor-cell D2DSS resource offset and a preset or signaled neighbor-cell discovery resource pool offset or ii) the difference between ‘SF#N+CEILING(w1)+1’ (or ‘SF#N+CEILING(w1)’) for configuring an INV_DL SF(s) related to neighbor-cell D2DSS reception and ‘starting SF-CEILING(w1)−1’ of a neighbor-cell discovery pool (or ‘starting SF−CEILING(w1)’ of the neighbor-cell discovery pool) for configuring an INV_DL SF(s) related to neighbor-cell discovery signal reception, as described in Example 1-1, is smaller than a preset or signaled threshold, it may be configured that a DL SF(s) overlapped in the time domain at least partially (i.e., partially or fully) with an area between ‘SF#N+CEILING(w1)+1’ (or ‘SF#N+CEILING(w1)’) for configuring an INV_DL SF(s) related to neighbor-cell D2DSS reception and ‘starting SF−CEILING(w1)−1’ of a neighbor-cell discovery pool (or ‘starting SF−CEILING(w1)’ of the neighbor-cell discovery pool) for configuring an INV_DL SF(s) related to neighbor-cell discovery signal reception, as described in Example 1-1, is also assumed/configured to be an INV_DL SF(S). This assumption/configuration may reduce frequent carrier (or frequency band) switchings of a single RX chain.

In an example, the assumption/configuration may be interpreted as meaning that if i) the difference between a preset or signaled neighbor-cell D2DSS resource offset and a preset or signaled neighbor-cell discovery resource pool offset or ii) the difference between ‘SF#N+CEILING(w1)+1’ (or ‘SF#N+CEILING(w1)’) for configuring an INV_DL SF(s) related to neighbor-cell D2DSS reception and ‘starting SF-CEILING(w1)−1’ of a neighbor-cell discovery pool (or ‘starting SF−CEILING(w1) of the neighbor-cell discovery pool’) for configuring an INV_DL SF(s) related to neighbor-cell discovery signal reception, as described in Example 1-1, is greater than a preset or signaled threshold, it may be configured that a DL SF(s) overlapped in the time domain at least partially (i.e., partially or fully) with an area between ‘SF#N+CEILING(w1)+1’ (or ‘SF#N+CEILING(w1)’) for configuring an INV_DL SF(s) related to neighbor-cell D2DSS reception and ‘starting SF−CEILING(w1)−1’ of a neighbor-cell discovery pool (or ‘starting SF−CEILING(w1) of the neighbor-cell discovery pool’) for configuring an INV_DL SF(s) related to neighbor-cell discovery signal reception, as described in Example 1-1, is not assumed/configured to be an INV_DL SF(S).

<Method 2 >

As described above, a (serving-cell/neighbor-cell) D2DSS resource with a preset or configured periodicity may be linked to a plurality of (serving-cell/neighbor-cell) D2D signal resource pools. Or one (serving-cell/neighbor-cell) D2DSS configuration may be used for a plurality of (serving-cell/neighbor-cell) D2D signal resource pools. Further, for example, the (serving-cell/neighbor-cell) D2DSS resource period may be fixed to 40 ms.

In consideration of this characteristic, when an INV_DL SF(s) related to reception of a (serving-cell/neighbor-cell) D2DSS(s) linked to a (serving-cell/neighbor-cell) D2D signal resource pool(s) is configured, the SRXCH_D2D RX UE may configure an INV_DL SF(s) according to Method #1, taking into account only a (preceding or following) (serving-cell/neighbor-cell) D2DSS linked to a (serving-cell/neighbor-cell) D2D signal resource pool that the SRXCH_D2D RX UE intends to actually receive (i.e., the latest SF of the D2DSS resource before the start of the discovery pool).

For example, this configuration/assumption may be interpreted as meaning that the SRXCH_D2D RX UE does not configure an INV_DL SF(s) in consideration of a linked (serving-cell/neighbor-cell) D2DSS preceding (or before) a (serving-cell/neighbor-cell) D2D signal resource pool that the SRXCH_D2D RX UE does not receive or does not want to receive. Application of the configuration/assumption may reduce excessive INV_DL SF(s) configuration caused by a (serving-cell/neighbor-cell) D2DSS.

Further, for example, the assumption/configuration may be applied restrictively to a case in which the SRXCH_D2D RX UE is instructed by predefined dedicated signaling (e.g., RRC signaling) (from the serving cell) to perform a (serving-cell/neighbor-cell) discovery signal reception only in a specific (serving-cell/neighbor-cell) D2D signal resource pool.

In another example, when an INV DL SF(s) related to reception of a (serving-cell/neighbor-cell) D2DSS(s) linked to a (serving-cell/neighbor-cell) D2D signal resource pool(s) is configured, the SRXCH_D2D RX UE may configure an INV_DL SF(s) according to Method #1, taking into account only a (preceding or following) (serving-cell/neighbor-cell) D2DSS linked to a (serving-cell/neighbor-cell) D2D signal resource pool (i.e., the latest SF of the D2DSS resource before the start of the discovery pool).

<Method 1#3>

If it is difficult to acquire synchronization to an associated/linked (serving-cell/neighbor-cell) D2D signal resource pool with reception of one (serving-cell/neighbor-cell) D2DSS, it may be defined that preset or signaled Q (serving-cell/neighbor-cell) D2SS(s) preceding (or before) the (serving-cell/neighbor-cell) D2D signal resource pool are received/used.

In this case, the SRXCH_D2D RX UE may be configured to configure an INV_DL SF(s) (according to the afore-described Method #1 or Method #2) in consideration of the preset or signaled Q (serving-cell/neighbor-cell) D2SS(s) preceding (or before) the (serving-cell/neighbor-cell) D2D signal resource pool.

Further, a method for setting usage indexes in relation to a discovery pool configuration may be defined as illustrated in [Table 5].

TABLE 5 The discovery pools RRC configuration can indicate a usage index per pool to reserve the pool for specific usages If more than 1 resource pool with the same usage index is configured for type 1 discovery, the network configures the method for the UE to select the resource pool among the pools with a given usage index; the following methods are supported: Random, subject to meeting the UE and network power configurations Default if no other method is configured UE RSRP measurement For each pool, an upper RSRP value and a lower RSRP value are configured For each value: {−infinity, −110 . . . −60, +infinity} dBm, increments of 10 dB

Methods for efficiently configuring an INV DL SF(s) (or a DL GAP) will be additionally described. Based on the afore-described [Table 4] and [Table 3], the operation of [Table 4] is valid as an operation for D2D discovery pools of a serving-cell D2D discovery pool and a D2D discovery pool of a neighbor cell for which a window length of w2 is indicated.

However, regarding pools of a neighbor cell for which a window length of w1 is indicated, a 1-ms margin preceding or following a discovery pool is not sufficient to accommodate the ambiguity of a cell timing. In other words, a DL gap should be defined as SFs belonging to discovery resource pools of the neighbor cell on a UL carrier, (cell(w1)+1) SFs preceding the SFs, and (cell(w1)+1) SFs following the SFs.

Further, a UE needs to receive a D2DSS in the first SF of the discovery pool or in an SF closest to the first SF of the discovery pool, before the discovery pool.

In consideration of this, a D2DSS SF related to a discovery pool of a neighbor cell, (cell(w1)+1) SFs preceding the D2DSS SF, and (cell(w1)+1) SFs following the D2DSS SF need to be configured as an (additional) DL gap.

FIG. 14 is a view referred for comparing DL gaps required for neighbor cells with synchronization window lengths of w2 and w1. FIG. 14(a) illustrates a DL gap required for a neighbor cell with a synchronization window length of w2, and FIG. 14(b) illustrates a DL gap required for a neighbor cell with a synchronization window length of w1.

Whether a DL gap for a discovery pool and an associated D2DSS SF is a single continuous DL gap or two independent (separate) DL gaps may be additionally considered. Also, whether a DL gap for a D2DSS SF can be configured for a D2DSS-incapable UE may be considered.

For example, a DL gap related to discovery may be applied to discovery pools of a serving cell or discovery pools of a neighbor cell for which a synchronization window length of w2 is indicated.

For a neighbor cell having a synchronization window length of w1, a DL gap may be configured for discovery pools, D2DSS SFs related to the discovery pools, (cell(w1)+1) SFs preceding the D2DSS SFs, and (cell(w1)+1) SFs following the D2DSS SFs.

Now, whether an eNB is capable of controlling a DL gap configuration for each UE will be described below. A DL gap may be unnecessary for some UEs according to the capabilities of the UEs and a CA configuration.

For example, if a UE is capable of both communication and discovery, and DL CA supporting simultaneous reception of a WAN DL signal and a D2D signal is configured for the UE, the UE may not need a DL gap for reception of a discovery signal.

Even though the UE does not support D2D communication, UE capability signaling indicating a condition (for the UE) requiring a DL gap (or indicating whether a DL gap is necessary) may be defined (refer to [Table 6]).

Accordingly, controllability on a DL gap for UE-specific discovery is needed to minimize DL SF loss.

If it is assumed that an eNB controls a DL gap, it should be determined whether to configure a DL gap for a specific resource pool and/or a specific cell. A UE may not be interested in receiving a discovery signal transmitted in a pool having a specific usage index. It may also be impossible for the UE to receive a discovery signal transmitted from a specific neighbor cell due to the distance from the cell.

Therefore, regarding configuration of a DL gap, the eNB may control a DL gap configuration in a pool-specific/neighbor cell-specific manner according to the present disclosure.

A description will be given of a method for performing a DL WAN operation during a DL gap by a UE having a single RX chain. For example, if the reception timing of a PHICH belongs to a DL gap, the UE may assume the PHICH as an ACK and report it to its higher layer in order to prevent an unintended PUSCH retransmission.

Moreover, if a CSI reference resource (e.g., SF#n) related to CSI reporting belongs to the DL gap, the CSI reference resource may be replaced with a valid DL SF closest to SF#n before SF#n(, which is not located in the DL gap). In another example, in this case, the UE may be defined to report a predefined CSI value. An operation related to a discontinuous reception (DRX) counter may also be defined. For example, since the UE may receive a PDCCH in another serving cell (i.e., another aggregated carrier) in which a D2D discovery signal is not received, the UE may maintain (or perform) DRX counting even in the DL gap.

<Method #4>

If an INV_DL SF(S) is configured according to the afore-described Method #1/Method #2/Method #3, it may be defined that the SRXCH_D2D RX UE conducts WAN communication according to at least a part (i.e., a part or all) of the following Example 4-1 to Example 4-3.

EXAMPLE 4-1

For example, if an interference measurement resource (IMR) related to calculation/derivation of (periodic/aperiodic) CSI reported at a specific time instant resides in an INV_DL SF, the SRXCH_D2D RX UE may be configured to assume that the IMR is not valid. It may be configured that a CSI report is transmitted/calculated (re)using an IMR included in a non-INV_DL SF closest to the INV_DL SF before the INV_DL SF, the CSI report is omitted, or CSI of a predefined specific value (e.g., out-of-range (OOR)) is reported.

For example, if a CSI reference resource related to calculation/derivation of (periodic/aperiodic) CSI reported at a specific time instant resides in an INV_DL SF, the SRXCH_D2D RX UE may be configured to assume that the CSI reference resource is not valid. It may be configured that the CSI report is transmitted/calculated (re)using/(re)assuming, as a CSI reference resource, a DL SF which is at once a non-INVL_DL SF closest to the INV DL SF (or the earliest non-INV_DL SF) before the INV_DL SF and a valid DL SF, the CSI report is omitted, or CSI set to a predefined value (e.g., OOR) is reported.

For example, application of the afore-described Example 4-1 may be interpreted as meaning that an INV_DL SF is not used for CSI measurement. The CSI measurement means at least one of desired signal measurement and/or interference measurement. In another example, it may be configured that not a D2D signal reception but a WAN communication-related CSI measurement operation is performed in the INV_DL SF. In another example, the INV_DL SF may be configured not to be used for radio resource management (RRM) and/or radio link monitoring (RLM).

EXAMPLE 4-2

If the afore-described Example 4-1 is applied, it may be configured that a CSI reference resource related to calculation of (periodic/aperiodic) CSI reported at a specific time instant is redetected only within a predefined or signaled time window (hereinafter, referred to as “WIN_SIZE”). As the time window is configured, excessive outdated CSI reporting may be reduced.

In a specific example, if SF#(R-4) being a CSI reference resource related to calculation/derivation of (periodic/aperiodic) CSI reported in SF#R is an INV_DL SF, a closest (or earliest) CSI reference resource before SF#(R-4), which is at once a non-INV_DL SF and a valid DL SF, is redetected only within an area ranging from ‘SF#(R-4−1)’ to ‘SF#(R-4−WIN_SIZE)’.

In another example, it may be configured that a valid IMR related to calculation of (periodic/aperiodic) CSI reported at a specific time instant is redetected only within a predefined or signaled time window. For example, if SF#(R-5) having an IMR related to calculation/derivation of (periodic/aperiodic) CSI reported in SF#R is an INV DL SF, a closest (or earliest) IMR before SF#(R-5), which satisfies the condition of an IMR located in a non-INV_DL SF, is redetected only within an area ranging from ‘SF#(R-5−1)’ to ‘SF#(R-5−WIN_SIZE)’ according to the foregoing configuration/assumption.

In the case where the above Example 4-2 is applied, if a valid CSI reference resource and/or a valid IMR does not exist or is not reselected in a re-search area based on a predefined or signaled time window, it may be configured that a corresponding CSI report is omitted or CSI set to a predefined specific value (e.g., OOR) is reported.

EXAMPLE 4-3

For example, if D2D communication and dynamic switching for radio resources (i.e., “EIMTA MODE”) are simultaneously configured for UE#Z having a single RX chain, and an SF related monitoring (or reception) of a dynamic switching indicator for radio resources (i.e., “EIMTA DCI”) is configured as an INV_DL SF, UE#Z may be configured not to perform EIMTA DCI monitoring (or reception) in the INV_DL SF. In another example, UE#Z may be configured to perform EIMTA DCI monitoring (or reception), not D2D signal reception, in the INV_DL SF.

In another example, as noted from [Table 3], reception of a predefined specific WAN DL signal is prioritized over at least one of i) (serving cell/neighbor cell-related) D2D signal reception, ii) (serving cell/neighbor cell-related) discovery signal reception, or iii) reception of a (serving-cell/neighbor-cell) D2DSS(s) linked to a (serving-cell/neighbor-cell) D2D signal resource pool(s). The WAN DL signal may be defined as a paging signal (and/or an SIB).

In the case where this definition is applied, if a D2D UE should receive a paging signal (and/or SIB) (SF#N) during one of reception of a (neighbor-cell/serving-cell) discovery signal in a (neighbor-cell/serving-cell) D2D signal resource pool and reception of a (neighbor-cell/serving-cell) D2DSS linked to the (neighbor-cell/serving-cell) D2D signal resource pool, at least one of i) a configuration that the D2D UE does not perform a discovery signal reception operation in an SF(s) in the (neighbor-cell/serving-cell) D2D signal resource pool, overlapped in the time domain at least partially (i.e., partially or fully) with an area ranging from ‘SF#N−1’ to ‘SF#N+1’ (or ‘SF#N’), and ii) a configuration that the D2D UE does not perform a D2DSS reception operation in a (neighbor-cell/serving-cell) D2DSS SF(s) (or a D2DSS resource), overlapped in the time domain at least partially (i.e., partially or fully) with the area ranging from ‘SF#N−1’ to ‘SF#N+1’ (or ‘SF#N’) may be applied.

For example, it may be configured that in ‘SF#N’ (or the area ranging from ‘SF#N−1’ to ‘SF#N+1’), i) at least one of PHICH reception, EIMTA DCI reception, random access response reception, reception of message 4 (i.e., a contention resolution message) (in a contention-based random access procedure), and PHICH reception related to (re)transmission of message 3 (i.e., a PUSCH) (in the contention-based random access procedure) (which are not performed/valid in an INV_DL SF(s) due to application of Method #4) is performed, and/or ii) at least one of an IMR and a CSI reference resource in ‘SF#N’ (or the area ranging from ‘SF#N−1’ to ‘SF#N+1’) is assumed to be valid. It may be configured that this configuration is applied restrictively to an SRXCH_D2D RX UE.

In another example, it may be configured that in ‘SF#N’ (or the area ranging from ‘SF#N−1’ to ‘SF#N+1’), i) PHICH reception, EIMTA DCI reception, random access response reception, reception of message 4 (i.e., a contention resolution message) (in a contention-based random access procedure), or PHICH reception related to (re)transmission of message 3 (i.e., a PUSCH) (in the contention-based random access procedure) is not allowed, and/or ii) an IMR and a CSI reference resource in ‘SF#N’ (or the area ranging from ‘SF#N−1’ to ‘SF#N+1’) is assumed to be invalid.

An example of a D2D UE capability/operation of simultaneously receiving a D2D signal (i.e., a UL spectrum)/WAN DL signal (i.e., a DL spectrum) under an FDD environment is illustrated in [Table 7].

TABLE 7 For communication, UE is able to receive simultaneously on the DL and UL spectrum of FDD carriers supporting D2D For discovery, UE may not be able to receive simultaneously on the DL and UL spectrum of FDD carriers supporting D2D Send LS to RAN2, RAN4, and SA2 RAN2: To investigate impact on UE capability signalling of restriction  on cellular operation when D2D is operated [Public safety UEs are assumed to be able to simultaneously  perform cellular on DL carrier and D2D on associated UL  carrier for FDD band] For discovery, non-public safety UE may not be able to receive  simultaneously on the DL and UL spectrum of FDD carriers  supporting D2D There is no simultaneous operation of CA and D2D required for  Rel-12 D2D communication if we assume 2 DL CA capable  UEs RAN4: RAN1 asks feasibility and implication of simultaneous reception  of cellular on DL spectrum and D2D associated UL spectrum  for FDD band RAN1 asks feasibility and implication of single receiver chain  switching between cellular spectrum and D2D reception  associated UL spectrum for FDD band SA2: [Public safety UEs are assumed to be able to simultaneously perform cellular on one carrier and D2D on another carrier]

<Method#5>

If at least a part (i.e., a part or all) of the following conditions are satisfied, an INV_DL SF(s) which is configured according to at least a part (i.e., a part or all) of the proposed methods (e.g., Method #1, Method #2, Method #3, and Method #4) may not be configured. Method #5 may be configured to be applied restrictively only when a D2D operation is performed in an FDD system environment (DL and UL spectrums of FDD carriers supporting D2D).

EXAMPLE 5-1

For example, if a D2D UE is receiving a D2D communication signal and a D2D discovery signal simultaneously in the same (UL) carrier (or (UL) spectrum), an INV_DL SF(s) may not be configured. This is because the D2D UE already has or is equipped with a D2D receiver for receiving a D2D communication signal according to “For communication, RANI assumes that UE is able to receive simultaneously on the DL and UL spectrum of FDD carriers supporting D2D” in [Table 7].

In another example, in the case where the D2D UE is capable of signaling whether an INV DL SF(s) needs to be configured, if the D2D UE signals that it is not necessary to configure an INV_DL SF(s), an INV_DL SF(s) may not be configured.

In another example, in the case where the D2D UE is capable of signaling whether a DL gap needs to be configured, if the D2D UE signals that it is not necessary to configure a DL gap, a DL gap may not be configured.

EXAMPLE 5-2

For example, if a D2D UE is not capable of supporting D2DSS (TX/RX), a D2DSS-related INV_DL SF(s) which is configured based on at least one of the afore-described Method #1, Method #2, Method #3, and Method #4 may not be configured. If this UE capability is signaled or reported, an eNB/network may be configured to indicate whether a D2DSS-related INV DL SF(s) is configured or not by a predefined signal (e.g., dedicated (RRC) signaling or an SIB).

EXAMPLE 5-3

For example, i) if a measurement of a (neighbor-cell) D2DSS linked to a preset or signaled neighbor-cell discovery pool is equal to or smaller than a preset or signaled threshold (i.e., it is determined that a neighbor cell is remote from a serving cell/D2D RX UE), and/or ii) if a (modified) RSRP value (or (modified) RSRQ value) of the neighbor cell is equal to or smaller than a preset or signaled threshold (i.e., it is determined that the neighbor cell is remote from the serving cell (or the D2D RX UE)), a D2DSS-related INV_DL SF(s) (or a DL gap) and/or a discovery pool-related INV_DL SF(s) (or a DL gap) which is configured based on at least a part (i.e., a part or all) of the afore-described proposed methods (e.g., Method #1 and/or Method #2 and/or Method #3 and/or Method #4) may not be configured.

In another example, i) if the measurement of the (neighbor-cell) D2DSS linked to the preset or signaled neighbor-cell discovery pool is equal to or greater than the preset or signaled threshold, and/or ii) if the (modified) RSRP value (or (modified) RSRQ value) of the neighbor cell is equal to or greater than the preset or signaled threshold, a D2DSS-related INV_DL SF(s) (or a DL gap) and/or a discovery pool-related INV_DL SF(s) (or a DL gap) which is configured based on at least a part (i.e., a part or all) of the afore-described proposed methods (e.g., Method #1 and/or Method #2 and/or Method #3 and/or Method #4) may not be configured.

For example, if the D2D UE reports to the eNB at least one of i) information indicating whether the measurement of the (neighbor-cell) D2DSS linked to the preset or signaled neighbor-cell discovery pool is equal to or less than the preset or signaled threshold, ii) information indicating whether the measurement of the (neighbor-cell) D2DSS linked to the preset or signaled neighbor-cell discovery pool is equal to or greater than the preset or signaled threshold, iii) information about the measurement of the (neighbor-cell) D2DSS linked to the neighbor-cell discovery pool, iv) information indicating whether the (modified) RSRP value (or (modified) RSRQ value) of the neighbor cell is equal to or less than the preset or signaled threshold, v) information indicating whether the (modified) RSRP value (or (modified) RSRQ value) of the neighbor cell is equal to or greater than the preset or signaled threshold, or vi) information about the (modified) RSRP value (or (modified) RSRQ value) of the neighbor cell, the serving cell receiving the information may indicate whether a D2DSS-related INV_DL SF(s) (or a DL gap) and/or a discovery pool-related INV_DL SF(s) (or a DL gap) is configured by a predefined UE-specific or cell-specific signal (e.g., dedicated (RRC) signaling or an SIB).

In another example, the eNB (or serving cell) may indicate whether a D2DSS SF(s) and/or a discovery (pool) SF(s) of a (neighbor) cell(s) is configured as an INV_DL SF(S) (or a DL gap), UE-specifically/cell-specifically.

In another example of the proposed method, it may be defined that the UE configures the above-described INV_DL SF(S) (or DL gap) only in a DL CC (or DL cell) linked to/paired with a (discovery) pool with an intended usage index, or the eNB configures the above-described INV_DL SF(S) (or DL gap) only in a DL CC (or DL cell) linked to/paired with a (discovery) pool with a specific usage index (or a specific (discovery) pool).

In another example of the proposed method, it may be configured that an INV_DL SF(s) (or a DL gap) is not configured for a cell for which synchronization error information of w1 (related to reception of a discovery signal and/or a D2DSS (linked to a discovery pool) is signaled, and at least one of i) discovery, ii) discovery signal reception, and iii) reception of a D2DSS (linked to the discovery pool) is performed in a best effort manner.

In another example of the proposed method, it may be configured that an INV_DL SF(s) (or a DL gap) is configured for a cell for which synchronization error information of w1 (related to reception of a discovery signal and/or a D2DSS (linked to a discovery pool) is signaled, like (or in the same manner as for) a cell for which synchronization error information of w2 (related to reception of a discovery signal and/or a D2DSS (linked to a discovery pool) is signaled, while the resulting performance degradation is allowed.

For example, for the cell for which the synchronization error information of w1 (related to reception of a discovery signal and/or a D2DSS (linked to a discovery pool) is signaled, it may be configured that i) a DL SF(s) overlapped in the time domain at least partially (i.e., partially or fully) with an area ranging from ‘(starting) SF-CEILING(w1)−1 of a neighbor-cell discovery pool’ to ‘(ending) SF+CEILING(w1)+1 of the neighbor-cell discovery pool’ is assumed to be an INV_DL SF(s) (or a DL gap), and/or ii) a DL SF(s) overlapped in the time domain at least partially (i.e., partially or fully) with an area ranging from ‘SF#N-CEILING(w1)−1’ to ‘SF#N+CEILING(w1)+1’ is assumed to be an INV_DL SF(s) (or a DL gap) (e.g., a neighbor-cell D2DSS resource is configured in SF#N of a serving cell), as described in Example 1-1 of Method #1.

On the other hand, for example, for the cell for which the synchronization error information of w2 (related to reception of a discovery signal and/or a D2DSS (linked to a discovery pool) is signaled, it may be configured that i) a DL SF(s) overlapped in the time domain at least partially (i.e., partially or fully) with an area ranging from ‘starting SF−1 of a neighbor-cell discovery pool’ to ‘ending SF+1 of the neighbor-cell discovery pool’ is assumed to be an INV_DL SF(s) (or a DL gap), and/or ii) a DL SF(s) overlapped in the time domain at least partially (i.e., partially or fully) with an area ranging from ‘SF#N−1’ to ‘SF#N+1’ is assumed to be an INV_DL SF(s) (or a DL gap) (e.g., a neighbor-cell D2DSS resource is configured in SF#N of a serving cell), as described in Example 1-1 of Method #1.

Now, a description will be given of methods for efficiently configuring the above-described INV_DL SF(s) (or DL gap(s)), in the case where a D2D RX UE having a single RX chain (hereinafter, referred to as an “SRXCH_D2D RX UE”) or a D2D RX UE having a D2D/cellular shared RX chain (hereinafter, referred to as an “SHRXCH D2D RX UE”) performs i) a D2D discovery signal reception operation in another (UL) carrier in an inter-frequency, or ii) a D2D discovery signal reception in another PLMN (UL) carrier based on inter-PLMN. The SHRXCH_D2D RX UE (or SRXCH_D2D RX UE) may be interpreted as a UE that alternately uses or shares its (relatively small number of RX chains or a single) RX chain for D2D RX and WAN DL RX. The following proposed methods may be extended to configuration of a single cell as well as CA.

[Table 8] below illustrates a WAN DL signal reception operation (i.e., INV_DL SF(s) (or DL gap(s)) configuration) that an SHRXCH_D2D RX UE (or SRXCH_D2D RX UE) assumes when receiving a D2D discovery signal.

TABLE 8 For FDD carriers: A UE with a shared D2D/cellular Rx chain (or a UE with a single Rx chain) and is receiving D2D discovery signals on an UL carrier is not expected to read DL signals on the DL carrier paired to such UL carrier during the subframes belonging to the D2D discovery pools on that UL carrier as well as one subframe preceding and following these subframes.

<Method #6>

If a UE (e.g., a D2D RX UE having a single RX chain) (or an SRXCH_D2D RX UE (e.g., a D2D RX UE having a shared D2D/cellular RX chain) performs i) a D2D discovery signal reception operation in another (UL) carrier in an inter-frequency, or ii) a D2D discovery signal reception in another PLMN (UL) carrier based on inter-PLMN, the afore-described INV_DL SF(s) (or DL gap(s)) may be defined based on at least a part (i.e., a part or all) of the following rules/configurations disclosed in Example 6-1 to Example 6-8. For example, the SHRXCH_D2D RX UE (or SRXCH_D2D RX UE) may be interpreted as a UE that alternately uses (or shares) its (relatively small number of RX chains or a single) RX chain for D2D RX and WAN DL RX. For example, the following proposed methods may be extended to configuration of a single cell as well as CA.

For the convenience of description of the proposed methods, it is assumed that an SHRXCH_D2D RX UE (or SRXCH_D2D RX UE) performs a D2D discovery signal reception in another (UL) carrier (or another PLMN (UL) carrier) in an inter-frequency (hereinafter, referred to as “DIFF_CC”) in a situation where two cells (i.e., primary cell #A (i.e., DL CC#A, UL CC#A) and secondary cell #B (i.e., DL CC#B, UL CC#B) have been configured.

Further, another (UL) carrier in an inter-frequency may be designated as an inter-frequency UL CC from the viewpoint of UL CC#A (SCell#B (UL CC#B/DL CC#B)) of PCell #A, or as an inter-frequency UL CC from the viewpoint of UL CC#B (PCell#A (UL CC#A/DL CC#A)) of SCell #B. p For example, Method #6 may also be extended to a case in which a D2D discovery signal reception operation is performed in DIFF_CC, with three or more cells (or a single cell) configured.

EXAMPLE 6-1

If an SHRXCH_D2D RX UE (or SRXCH_D2D RX UE) performs a D2D discovery signal reception in DIFF_CC, the SHRXCH_D2D RX UE may configure an INV_DL SF(s) (or DL gap(s)) in all DL CC(s) (e.g., DL CC#A and DL CC#B) configured for the SHRXCH_D2D RX UE (or SRXCH_D2D RX UE) by CA. Application of this configuration/rule may be interpreted as meaning that whether to configure an INV_DL SF(s) (or DL gap(s)) in a specific (serving cell-related) DL CC depends on not whether the specific (serving cell-related) DL CC and DIFF_CC are in a paired DL CC relationship but whether a D2D discovery signal is received in DIFF_CC.

EXAMPLE 6-2

If the SHRXCH_D2D RX UE (or SRXCH_D2D RX UE) performs a D2D discovery signal reception in DIFF CC, the SHRXCH_D2D RX UE may configure an INV_DL SF(s) (or DL gap(s)) only in a predefined or signaled serving cell-related DL CC(s) among DL CC(s) (or serving cell(s)) configured for the SHRXCH_D2D RX UE (or SRXCH_D2D RX UE) by CA.

Herein, it may be interpreted that i) (virtual) pairing is configured between a DL CC(s) in which an INV_DL SF(s) (or DL gap(s)) is configured and DIFF_CC, or ii) a DL CC(s) in which an INV_DL SF(s) (or DL gap(s)) is configured is (virtually) paired with DIFF_CC.

When the SHRXCH_D2D RX UE (or SRXCH_D2D RX UE) performs a D2D discovery signal reception in DIFF_CC, i) information about a DL CC(s) or ii) a serving cell(s) in which an INV_DL SF(s) (or DL gap(s)) is configured may be signaled/defined as a pair/combination of “information about a (UL) CC (or serving cell) in which a D2D discovery pool is configured” and “information about a DL CC(s) (or serving cell(s)) in which an INV_DL SF(s) (or DL gap(s)) is configured, when the D2D discovery signal reception is performed in the (UL) CC (or serving cell) in which the D2D discovery pool is configured”.

EXAMPLE 6-3

It may be defined that when the SHRXCH_D2D RX UE (or SRXCH_D2D RX UE) performs a D2D discovery signal reception operation in DIFF_CC, the SHRXCH_D2D RX UE (or SRXCH_D2D RX UE) configures an INV_DL SF(s) (or a DL gap(s)) only in a DL CC (e.g., DL CC #A) of a PCell from among a DL CC(s) configured for the SHRXCH_D2D RX UE (or SRXCH_D2D RX UE) by CA. Application of this method may be interpreted as meaning that whether an INV_DL SF(s) (or a DL gap(s)) is configured in a DL CC (e.g., DL CC #A) of a PCell is determined according to whether a D2D discovery signal reception operation is performed in DIFF_CC, not whether a D2D discovery signal reception operation is performed in a UL CC (e.g., UL CC #A) paired with the PCell.

EXAMPLE 6-4

It may be defined that when the SHRXCH_D2D RX UE (or SRXCH_D2D RX UE) performs a D2D discovery signal reception operation in DIFF CC, the SHRXCH_D2D RX UE (or SRXCH_D2D RX UE) configures an INV_DL SF(s) (or a DL gap(s)) only in a DL CC(s) (e.g., DL CC #B) of an SCell(s) except for a PCell from among a DL CC(s) configured for the SHRXCH_D2D RX UE (or SRXCH_D2D RX UE) by CA. Application of this method may be interpreted as meaning that whether an INV_DL SF(s) (or a DL gap(s)) is configured in a DL CC(s) (e.g., DL CC #B) of an SCell(s) is determined according to whether a D2D discovery signal reception operation is performed in DIFF_CC, not whether a D2D discovery signal reception operation is performed in a UL CC(s) (e.g., UL CC #B) paired with the SCell(s).

EXAMPLE 6-5

If w1 is signaled as synchronization error information related to a discovery pool in DIFF_CC and/or reception of a D2DSS (linked to the discovery pool) in the foregoing examples (e.g., Example 6-1, Example 6-2, Example 6-3, and Example 6-4), i) a DL SF(s) of a DL CC(s) selected or indicated based on the above-described examples, which is overlapped in the time domain at least partially (i.e., partially or fully) with an area ranging from ‘(starting) SF−CEILING(w1)−1 of the DIFF_CC discovery pool’ to ‘(ending) SF+CEILING(w1)+1 of the DIFF_CC discovery pool’ is assumed to be an INV_DL SF(s) (or DL gap), and/or ii) a DL SF(s) of the DL CC(s) selected or indicated based on the above-described examples, which is overlapped in the time domain at least partially (i.e., partially or fully) with an area ranging from ‘SF#N-CEILING(w1)−1’ to ‘SF#N+CEILING(w1)+1’ is assumed to be an INV_DL SF(s) (or a DL gap) (i.e., a DIFF_CC D2DSS resource is configured in serving-cell SF#N). It may be defined that this configuration/rule is applied restrictively to a case in which DIFF_CC is for a neighbor cell (and/or serving-cell) and/or another PLMN (UL) carrier).

In another example, if w2 is signaled as synchronization error information related to a discovery pool in DIFF_CC (and/or reception of a D2DSS linked to the discovery pool), i) a DL SF(s) of a DL CC(s) selected or indicated based on the above-described examples, which is overlapped in the time domain at least partially (i.e., partially or fully) with an area ranging from ‘(starting) SF−1’ of the DIFF_CC discovery pool’ to ‘(ending) SF+1 of the DIFF_CC discovery pool’ is assumed to be an INV_DL SF(s) (or a DL gap) is assumed to be an INV_DL SF(s) (or a DL gap), and/or ii) a DL SF(s) of the DL CC(s) selected or indicated based on the above-described examples, which is overlapped in the time domain at least partially (i.e., partially or fully) with an area ranging from ‘SF#N−1’ to ‘SF#N+1’ is assumed to be an INV_DL SF(s) (or a DL gap) (i.e., a DIFF CC D2DSS resource is configured in serving-cell SF#N). It may be defined that this configuration/rule is applied restrictively to a case in which DIFF_CC is for a neighbor cell (and/or serving-cell) and/or another PLMN (UL) carrier)).

EXAMPLE 6-6

In the foregoing examples (e.g., Example 6-1, Example 6-2, Example 6-3, and Example 6-4), i) a DL SF(s) of a DL CC(s) selected or indicated based on the above-described examples, which is overlapped in the time domain at least partially (i.e., partially or fully) with an area ranging from ‘(starting) SF−1’ of the DIFF_CC discovery pool’ to ‘(ending) SF+1 of the DIFF_CC discovery pool’ is assumed to be an INV_DL SF(s) (or a DL gap), and/or ii) a DL SF(s) of the DL CC(s) selected or indicated based on the above-described examples, which is overlapped in the time domain at least partially (i.e., partially or fully) with an area ranging from ‘SF#N−1’ to ‘SF#N+1’ is assumed to be an INV_DL SF(s) (or a DL gap) (i.e., a DIFF_CC D2DSS resource is configured in serving-cell SF#N). It may be defined that this configuration/rule is applied restrictively to a case in which DIFF_CC is for a serving cell (and/or neighbor-cell) and/or another PLMN (UL) carrier)).

EXAMPLE 6-7

A serving eNB/D2D UE may be configured to indicate to a (another) D2D UE by a predefined signal (e.g., an SIB, a (dedicated) RRC signal, or a PD2DSCH), at least one of i) information indicating whether (a part or all of) the above-described example(s) (e.g., Example 6-1, Example 6-2, Example 6-3, Example 6-4, Example 6-5, and Example 6-6) is applied, ii) information indicating which one(s) of the foregoing examples is applied to which (serving) cell, iii) information indicating whether an INV_DL SF(s) (or a DL gap(s)) is configured due to a D2D discovery signal reception operation in another (UL) carrier in an inter-frequency, or iv) information indicating whether an INV_DL SF(s) (or a DL gap(s)) is configured due to a D2D discovery signal reception operation in another PLMN (UL) carrier, or the at least one piece of information may be preset.

In another example, the above-described example(s) (e.g., Example 6-1, Example 6-2, Example 6-3, Example 6-4, Example 6-5, and Example 6-6) may be extended to a case in which an SHRXCH_D2D RX UE (or SRXCH_D2D RX UE) performs a D2D communication signal (e.g., SA or D2D data) reception operation in DIFF_CC.

In another example, if there is a DL CC paired with DIFF_CC in the above-described example(s) (e.g., Example 6-1, Example 6-2, Example 6-3, Example 6-4, Example 6-5, and Example 6-6) (e.g., a D2D discovery signal reception operation is performed in another (UL) carrier of an inter-frequency), and the SHRXCH_D2D RX UE (or SRXCH_D2D RX UE) performs a D2D discovery signal reception operation in DIFF_CC, an INV_DL SF( ) (or a DL gap(s)) (based on the afore-described rule) may be configured i) always or ii) only when configuration of an INV_DL SF( ) (or a DL gap(s)) is enabled.

EXAMPLE 6-8

Information about an INV_DL SF( ) (or a DL gap(s)) configured when the SHRXCH_D2D RX UE (or SRXCH_D2D RX UE) performs a D2D discovery signal reception operation in DIFF_CC may be indicated or signaled in the form of a bitmap. The serving eNB (or D2D UE) may indicate information about a (serving) cell to which the information is to be actually applied by an additional signal (e.g., an SIB, a (dedicated) RRC signal, or a PD2DSCH), or the information may be set based on a preset rule or at least a part (i.e., a part or all of) of the above-described (predefined or signaled) rules (e.g., Example 6-1, Example 6-2, Example 6-3, Example 6-4, Example 6-5, and Example 6-6).

<Method #7>

If an INV_DL SF(s) (or a DL gap(s)) is configured according to at least a part (i.e., a part or all of) of the above-described methods (e.g., Method #1, Method #2, Method #3, Method #4, and Method #5), a DRX operation may be performed according to at least a part (i.e., a part or all of) of the following configurations.

Before a detailed description of the present disclosure, a legacy (UE) DRX configuration/operation is performed as illustrated in [Table 9].

Herein, DRX configurations for individual UEs may be set UE-specifically depending on whether a serving cell has DL data (to be transmitted) for each UE. Further, Method #7 may be applied restrictively to an SRXCH_D2D RX UE. Also, Method #7 may be applied restrictively to a case in which an RRC_CONNECTED UE (or RRC_IDLE UE) performs a DRX operation. Further, Method #7 may be applied restrictively to a case in which CA is applied (and/or a case in which CA is not applied).

EXAMPLE 7-1

It may be configured that predefined or signaled DRX operation-related specific timers (e.g., onDurationTimer, drx-InactivityTimer, and drx-RetransmissionTimer) do not count an INV_DL SF(s) configured based on at least a part (i.e., a part or all) of the afore-described proposed methods (e.g., Method #1, Method #2, Method #3, Method #4, and Method #5). Herein, the INV_DL SF(S) may be interpreted as a kind of measurement gap (i.e., [Table 9]) that the DRX operation-related specific timers do not count in a legacy operation, or may be interpreted as an SF(s) other than a PDCCH SF(s) (refer to [Table 9]).

EXAMPLE 7-2

It may be configured that predefined or signaled DRX operation-related specific timers (e.g., onDurationTimer, drx-InactivityTimer, and drx-RetransmissionTimer) count an INV_DL SF(s) configured based on at least a part (i.e., a part or all) of the afore-described proposed methods (e.g., Method #1, Method #2, Method #3, Method #4, and Method #5). If this method is applied, the INV DL SF(S) may be interpreted as an SF(s) in which PDCCH monitoring is not actually performed but which the predefined or signaled DRX operation-related specific timers count.

EXAMPLE 7-3

Predefined or signaled DRX operation-related specific timers (e.g., onDurationTimer, drx-InactivityTimer, and drx-RetransmissionTimer) may count in different manners for CA and for non-CA.

For example, it is assumed for the convenience of description that two cells (e.g., [CELL#1]: DL CC#A, UL CC#A, [CELL#2]: DL CC#B, UL CC#B) are configured by CA, and a D2D signal (e.g., (serving/neighbor) discovery signal) reception operation is configured/performed only in UL CC#A of CELL#1. In this situation, the predefined or signaled DRX operation-related specific timers may be configured to count an INV_DL SF(s) configured based on the above-described at least part (i.e., part or all) of the methods. This is because a WAN DL signal reception operation is not performed only in DL CC#A (paired with UL CC#A) due to a D2D signal reception operation in UL CC#A of CELL#1 (refer to [Table 3]), but a WAN DL signal reception operation is still possible in DL CC#B of CELL#2, and a DRX operation is commonly applied/performed irrespective of cells.

On the other hand, in another example, if CA is not applied, the predefined or signaled DRX operation-related specific timers may be configured not to count an INV_DL SF(s) configured based on the above-described at least part (i.e., part or all) of the methods. This is because if CA is not applied, there are no other cells in which a WAN DL signal reception operation can be performed in the INV_DL SF(s).

In another example, if CA is applied, the predefined or signaled DRX operation-related specific timers may be configured not to count an INV_DL SF(s) configured based on the above-described at least part (i.e., part or all) of the methods, whereas if CA is not applied, the predefined or signaled DRX operation-related specific timers may be configured to count an INV_DL SF(s) configured based on the above-described at least part (i.e., part or all) of the methods.

Method #8

The above-described INV_DL SF(s) (or DL gap(s)) may be configured according to at least a part (i.e., a part or all) of rules disclosed in at least one of the following Example 8-1 to Example 8-3.

Before a detailed description of this method, an example of a legacy (UE) random access procedure is illustrated in [Table 10] and [Table 11].

It may be configured that Method #8 is applied restrictively to an SRXCH_D2D RX UE. It may also be configured that Method #8 is applied restrictively to a case in which an RRC_CONNECTED UE (or RRC_IDLE UE) performs a random access procedure. It may also be configured that Method #8 is applied restrictively to CA (and/or non-CA). It may also be configured that Method #8 is applied restrictively to a contention-based random access procedure (and/or a contention free-based random access procedure).

For the convenience of description, a set of INV_DL SFs (or DL gaps) (and/or a set of D2DSS-related INV_DL SFs (or DL gaps) (linked to a corresponding discovery pool)) configured in at least a part (i.e., a part or all) of the above-described proposed methods (e.g., Method #1, Method #2, Method #3, Method #4, Method #5, and Method #6) is referred to as “INV_DL SF(s)_DISWIN” (and/or “INV_DL SF(s)_DSSWIN”). For example, it may be configured that Method #8 is applied restrictively to “INV_DL SF(s)_DISWIN” (and/or “INV_DL SF(s)_DSSWIN”).

EXAMPLE 8-1

If INV_DL SF(s)_DISWIN (and/or INV_DL SF(s)_DSSWIN) is overlapped at least partially (i.e., partially or fully) with the afore-described random access response window (refer to [Table 11]), a D2D UE may be configured not to configure INV_DL SF(s)_DISWIN (and/or INV_DL SF(s)_DSSWIN) related to a corresponding discovery pool (and/or a D2DSS linked to the discovery pool) (i.e., not to receive a discovery signal and/or a D2DSS linked to the discovery pool).

Herein, application of this configuration/rule may be interpreted as meaning that reception of a random access response (or a random access procedure) is prioritized over reception of a discovery signal (and/or a D2DSS linked to a discovery pool).

EXAMPLE 8-2

If INV_DL SF(s)_DISWIN (and/or INV_DL SF(s)_DSSWIN) is overlapped at least partially (i.e., partially or fully) with the afore-described random access response window (refer to [Table 11]), the D2D UE may be configured to perform a random access response reception operation in an area ranging from ‘(starting) SF of a random access response window’ to ‘(ending) SF of the random access response window’ (i.e., not to receive a discovery signal and/or a D2DSS linked to the discovery pool), and not to receive a discovery signal and/or a D2DSS linked to the discovery pool (i.e. to perform a WAN DL signal reception operation) in an SF(s) following ‘the ending SF of the random access response window’.

For example, in the case where this rule is applied, if there is also a (reception) resource related to a discovery pool (and/or a D2DSS linked to the discovery pool) before the starting SF of the random access response window’, it may be configured that an operation for receiving a discovery signal and/or a D2DSS linked to the discovery pool is not performed in resources overlapped at least partially (i.e., partially or fully) with ‘the starting SF−1 of the random access response window’ (i.e., an SF(s) that ensures a time required for carrier (or frequency band) switching of a single RX chain). For example, if an SF(s) preceding an SF overlapped at least partially (i.e., partially or fully) with ‘the starting SF−1 of the random access response window’ is also configured as a discovery pool and/or a D2DSS-related (reception) resource linked to the discovery pool, it may be configured that an operation for receiving a discovery signal and/or a D2DSS linked to the discovery pool is performed in the preceding SF(s).

EXAMPLE 8-3

If INV_DL SF(s)_DISWIN (and/or INV_DL SF(s)_DSSWIN) is overlapped at least partially (i.e., partially or fully) with the afore-described random access response window (refer to [Table 11]), the D2D UE may be configured not to perform an operation for receiving a discovery signal and/or a D2DSS linked to a discovery pool, only in an SF(s) overlapped at least partially (i.e., partially or fully) with an area ranging from ‘the starting SF−1 of the random access response window’ to ‘the ending SF+1 of the random access response window’.

EXAMPLE 8-4

If INV_DL SF(s)_DISWIN (and/or INV_DL SF(s)_DSSWIN) is overlapped with an SF(s) (a set of SFs) (hereinafter, referred to as “WAN_WIN”) for at least a part (i.e., a part or all) of the following usages, the D2D UE may not configure INV_DL SF(s)_DISWIN (and/or INV_DL SF(s)_DSSWIN) (related to a discovery pool and/or a D2DSS linked to the discovery pool).

In another example, if INV_DL SF(s)_DISWIN (and/or INV_DL SF(s)_DSSWIN) is overlapped at least partially (i.e., partially or fully) with the above-described WAN_WIN, the D2D UE may be configured to perform a WAN_WIN-related reception operation (i.e., not to receive a discovery signal and/or a D2DSS linked to a discovery pool) in an area ranging from the starting SF of WAN_WIN’ to ‘the ending SF of WAN_WIN’, and not to receive a discovery signal and/or a D2DSS linked to a discovery pool (i.e., to perform a WAN DL signal reception operation) in an SF(s) following ‘the ending SF of WAN_WIN’.

Herein, in the case where this configuration/rule is applied, if there is also a (reception) resource related to a discovery pool and/or a D2DSS linked to the discovery pool before ‘the starting SF of WAN_WIN’, it may be configured that an operation for receiving a discovery signal and/or a D2DSS linked to the discovery pool is not performed in resources overlapped at least partially (i.e., partially or fully) with ‘the starting SF−1 of WAN_WIN−1’ (i.e., an SF(s) for ensuring a time required for carrier (or frequency band) switching of a single RX chain). For example, if an SF(s) preceding an SF overlapped at least partially (i.e., partially or fully) with ‘the starting SF−1 of WAN_WIN’ is configured as (reception) resources related to the discovery pool and/or the D2DSS linked to the discovery pool, an operation for receiving a discovery signal and/or a D2DSS linked to the discovery pool is performed in the preceding SF(s).

In another example, if INV_DL SF(s)_DISWIN (and/or INV_DL SF(s)_DSSWIN) is overlapped at least partially (i.e., partially or fully) with the above-described WAN_WIN, the D2D UE may be configured not to perform an operation for receiving a discovery signal and/or a D2DSS linked to a discovery pool, only in an SF(s) overlapped at least partially (i.e., partially or fully) with an area ranging from the starting SF−1 of WAN_WIN’ to ‘the ending SF+1 of WAN_WIN’.

-   a random access response window -   a (re)transmission time instant of message 3 (e.g., a PUSCH) (in a     contention-based random access procedure) -   a reception time instant of a PHICH related to (re)transmission of     message 3 (e.g., a PUSCH) (in the contention-based random access     procedure) -   a (re)transmission time instant of message 4 (i.e., a contention     resolution message) (in the contention-based random access     procedure) -   a (re)transmission time instant of a random access preamble

<Method 9>

Further, a description will be given of a method for efficiently supporting D2D discovery in a multi-carrier situation to which the present disclosure is applicable, or a method for efficiently supporting D2D discovery in a carrier other than a PCell.

[Table 12] describes a method for supporting discovery, or a method for efficiently supporting D2D discovery in a carrier other than a PCell.

TABLE 12 1. Introduction One objective of Rel-13 eD2D WI is to enhance D2D discovery in multiple carriers [1]: 3) Enhance D2D discovery support in the presence of multiple carriers and PLMNs: a) Allow D2D transmissions in a non-serving carrier and/or secondary cell belonging to the same and possibly different PLMN as the serving cell. In order to fulfil the above objective, it needs to be defined how the UE can know the TX and RX resource pools in other carriers as well as how the UE utilizes its limited TX and RX capability in the multi-carrier scenarios. This contribution focuses on these two aspects. For brevity, a carrier which is not PCell of a UE is called non-PCell throughout this paper, and a non-PCell can be an SCell, a non-serving carrier belonging to the same PLMN as PCell, or a carrier belonging to a different PLMN. 2. Resource pool configuration In Rel-12 discovery, the eNB signals the configuration of TX and RX resource pools only for the carrier in which this signaling is transmitted. For the other carriers, the eNB only can inform the list of carriers on which discovery may be operated, and if a UE is interested in discovery monitoring in the other carriers, the UE may read SIB from other carriers to acquire the resource pool configuration. However, such Rel-12 inter-carrier discovery monitoring is operated in a best-effort basis, and the eNB has no idea about when the UE actually operates the inter-carrier discovery. It is our understanding that the Rel-13 objective of the multi-carrier discovery is to define more predictable UE behavior and related performance requirement, especially for discovery transmissions which consumes radio resources and causes interference. In order to achieve this objective, it is necessary to have a clear definition of the resource pool in a non-PCell so that the UE behavior about the pool can be explicitly specified in a predictable manner. Therefore, the eNB needs to signal TX and RX resource pools for a non-PCell on which the related UE behavior can be clearly defined.

Regarding the case of [Table 12], an eNB may signal a TX/RX discovery resource configuration for a carrier (or cell) other than a PCell through the PCell according to the present disclosure.

For example, in the case of intra-PLMN, a discovery transmission (resource) configuration in another carrier may be allowed for a UE by RRC signaling. For example, RRC signaling may be used for the usage of configuring type 1 or type 2 discovery (resources) in a non-primary carrier.

In another example, in the case of inter-PLMN, a higher layer may handle inter-PLMN authentication for discovery signal transmission. In the presence of a network infrastructure, a network having inter-PLMN information should have an option for making a (discovery transmission (resource)) configuration for a UE, similarly to intra-PLMN. Since inter-PLMN coordination is not possible all the time, uncoordinated inter-PLMN should be considered basically. If an uncoordinated inter-PLMN scenario with a network infrastructure is considered, a UE may be aware of TX/RX resource pool information to be used (in a corresponding carrier frequency) by parsing (or detecting) SIB19 of a concerned carrier frequency. Herein, for example, a carrier frequency (in which a D2D discovery signal is to be transmitted(/received)) may be indicated to the UE by predefined signaling (or rule). On the other hand, an inter-PLMN scenario without a network infrastructure (e.g., a case in which an eNB is not on a ProSe (or D2D) carrier) may be supported, and out-of-coverage discovery may also be considered to be supported.

Further, transmission of a non-PCell will be described. First, how a UE will determine a transmission synchronization reference will be described.

FIG. 15 illustrates coverage states in a multi-carrier environment related to the present disclosure. FIG. 15(a) illustrates a case in which a TX UE is located within the network coverage of a PCell and within the network coverage of a concerned non-PCell, and FIG. 15(b) illustrates a case in which a TX UE is located within the network coverage of a PCell but outside the network coverage of a concerned non-PCell.

FIG. 16 illustrates a heterogeneous network in a multi-carrier environment. Referring to FIG. 16, even though CA is applied, the timings of a PCell and a non-PCell may be different (e.g., in the case of multiple TAGs) due to a propagation delay difference or the like in a heterogeneous network environment (refer to FIG. 16). For example, in the case illustrated in FIG. 15(a), when a UE transmits a D2D signal in a non-PCell, the UE preferably uses a cell of the non-PCell as a transmission synchronization reference. For this purpose, an eNB needs to signal a synchronization reference ID related to each TX pool in the non-PCell, similarly to configuring (or signaling) RX pools for neighbor cells.

For example, the difference (from configuring (or signaling) RX pools for neighbor cells) lies in that a resource pool-related bitmap is applied based on SFN#0 of an indicated reference cell located in a non-PCell carrier. Upon receipt of this information, the TX UE is synchronized with the indicated reference cell and transmits a D2D signal according to a resource pool configuration. The indicated cell is a reference for all transmissions in the resource pool (e.g., RSRP-based SLSS transmission, RSRP-based resource pool selection, path-loss dependent open-loop power control).

If the UE is in RRC_Connected state in the PCell, a single reference cell-based TX pool may be sufficient. This is because the network may select a reference cell based on an RRM report received from the UE. If the UE is already connected to a cell on a non-PCell according to CA or dual connectivity, a reference cell ID may not be indicated because the connected cell may become a transmission synchronization reference (naturally). However, if the UE is in RRC_Idle state, a plurality of small cells may exist on the non-PCell carrier within the coverage of the PCell, as illustrated in FIG. 16. Therefore, in this case, PCell signaling (e.g., an SIB) may need a plurality of TX pools related to each reference cell. Further, a reference cell selection procedure in an appropriate non-PCell carrier needs to be defined.

According to the present disclosure, therefore, the PCell needs to indicate the ID of a transmission synchronization resource for each discovery resource pool in a carrier other than the PCell. For RRC_Idle UEs, the PCell may configure a plurality of TX pools for a carrier other than the PCell. In the latter case (e.g., in the case of RRC_Idle UEs), one TX pool may be selected by a proper selection procedure.

Further, in the case of FIG. 15(b), in-coverage and out-coverage are co-existent in carriers in the D2D synchronization procedure context, from the viewpoint of a single UE. Therefore, in this case, i) whether the UE is to operate as an out-coverage UE, ii) whether the UE is to operate as an in-coverage UE, or iii) whether a new operation is to be defined should be considered.

First, the case where the UE operates as an out-coverage UE in a non-PCell according to a synchronization procedure that defines a network coverage state on a carrier basis (Option 1) will be described. In this case, the network does not have controllability on a D2D operation on the non-PCell, and it is very difficult for a WAN signal of the PCell and a D2D signal of a non-PCell to coexist. For example, if a UE (e.g., D2D UE#X) is synchronized with an SLSS from another out-coverage UE already existing in the non-PCell, the position of a D2D operation is not known (accurately) to the eNB. As a result, TDM between WAN TX and D2D TX in two carriers may be impossible. To avert this problem, D2D UE#X may be configured to indicate to the PCell at least one of i) resource configuration/position information about an SLSS/PSBCH, ii) resource configuration/position information about a PSSCH, iii) resource configuration/position information about a PSCCH, iv) resource configuration/position information about a PSDCH, or v) information about the time/frequency synchronization difference between PCell communication (e.g., D2D communication or WAN communication) and out-coverage D2D communication (non-PCell) for use in D2D communication with an out-coverage D2D UE (non-PCell) (acquired/discovered by D2D UE#X), through a predefined channel (or resource).

In the case where the network still has controllability on a D2D operation in a non-PCell (Option 2), the network may configure resources and a sequence for transmission of an SLSS on the non-PCell by signaling on a PCell. By this control, the network may determine the location of the D2D operation on the non-PCell and enable efficient coexistence with WAN communication. For synchronized transmission, i) when a UE transmits an in-coverage synchronization signal (e.g., an SLSS sequence) (as an in-coverage UE), the UE may assume(/determine) that the synchronization signal has priority over an SLSS transmitted by another out-coverage UE on in a non-PCell carrier. In another example, for synchronized transmission, ii) the UE may transmit a synchronization signal (e.g., an SLSS sequence) (as an out-coverage UE), ignoring an SLSS transmitted by another out-coverage UE in a non-PCell carrier.

The above-described operations (i.e., Option 1 and Option 2) may be applied simultaneously (or co-exist). For example, if synchronization resources are not configured in a non-PCell, Option 1 may be applied, whereas if synchronization resources are configured in a non-PCell, Option 2 may be applied.

Now, reception in a non-PCell will be described. For an RX pool configured in the non-PCell, a reference cell should also be considered. SFN#0 of a cell that provides a signal in relation to discovery is a reference for all SF bitmaps for resource pool configuration.

A synchronization window (w1 or w2) may be signaled for each pool. Therefore, a UE may be synchronized with a resource pool, assuming that a timing error (with respect to a signaling cell) is limited to within the indicated synchronization window.

To reuse a resource pool structure for an RX operation in a non-PCell, an RX reference cell should be determined. A PCell (i.e., a signaling cell) or a cell of a non-PCell may be a reception reference cell.

In a network with dual connectivity, the PCell may not be synchronized with the cell of the non-PCell. If the PCell is used as a reception reference cell of the non-PCell, it may be indicated that all RX pools have a large synchronization window (i.e., w1) irrespective of synchronization states with respect to the cell of the non-PCell.

As a result, if additional information is not provided, a UE should repeat RX pool timing detection. This unnecessary operation may be prevented by allowing additional information by which to consider that cells sharing the same SLSS ID are time-synchronized, or providing a time-synchronized cell list. Alternatively, a cell or a set of synchronized cells in a non-PCell carrier may become a reception reference cell. After receiving a resource configuration in the PCell, the UE may perform an operation other than indication of a cell operating as a reference instead of the PCell.

UE TX/RX capabilities will further be described below.

If a UE does not have a D2D-specific TX/RX circuit, the UE should switch a circuit for WAN to serve a D2D purpose. This switching is based on an FDD PCell (i.e., the UE switches a WAN receiver from a DL carrier to a UL carrier in order to receive a discovery signal). Therefore, additional UE capability signaling may be configured to accurately indicate the UE's simultaneous TX/RX capabilities in multiple carriers to the network, in consideration of UE complexity.

That is, according to the present disclosure, since a UE having a D2D-specific TX/RX circuit is not defined, UE capability signaling should be defined to indicate accurate multiple carriers-related simultaneous TX/RX capabilities to the network.

Further, [Table 13] describes a configuration of WAN and D2D carriers, in the case where a UE-related TX/RX limit is exceeded.

TABLE 13  A proper solution is needed to handle the case where the configuration of carriers for WAN and D2D exceeds the limitation in TX and RX of the UE. In Rel-12, the concept of “ProSe gap” was agreed in RAN1 but not included in the final specification due to the lack of time to resolve the related issues. By the following ProSe gap, a UE can stop receiving WAN in a carrier (DL carrier in the agreement) during the gap and switch the receiver to another carrier (UL carrier in the agreement) for discovery reception. For FDD carriers: At least for UEs with a single Rx chain (FFS subject to the UE capability discussion whether this also applies for UEs with a shared D2D/cellular Rx chain), a UE that is receiving D2D discovery signals on an UL carrier is not expected to read DL signals on the DL carrier paired to such UL carrier during the subframes belonging to the D2D discovery pools on that UL carrier as well as one subframe preceding and following these subframes The discovery pools are configured by the eNB by broadcast or UE- specific signaling FFS: For RRC_CONNECTED UEs, 1 bit may be signalled using RRC signaling indicating whether this rule applies or not (on a per UE basis) Cellular measurement gaps subframes are excluded from this rule Paging reception is prioritized over D2D reception

The present disclosure proposes a ProSe gap for efficient TDM between WAN and D2D discovery for a UE having a limited RF capability, based on the configuration described in [Table 13].

The UE discontinues monitoring of WAN DL reception and starts to receive a discovery signal in another carrier (a related UL carrier or an unrelated carrier), in a ProSe gap. The ProSe gap is required for a carrier switching time and a discovery reception operation including SLSS reception.

The ProSe gap is also required when the UE switches a single TX circuit between WAN UL TX on a PCell and discovery TX on a non-PCell. That is, it may be determined that a DL reception linked to a UL SF included in the ProSe gap for Tx is disabled.

For example, a (UL) gap attributed to D2D TX (non-PCell) may not be configured/allowed in a WAN UL (TX) SF (PCell) linked to a DL SF(s) (PCell) carrying a (DL) signal/channel/data for a predefined usage (e.g., paging, RAR, SIB, PSS?SSS, or PSBCH). Herein, ‘a DL SF(s) linked to a WAN UL (TX) SF’ may be interpreted as a DL SF(s) carrying a PUSCH transmission-related UL grant (and/or a PHICH) (according to a predefined or signaled UL HARQ timeline).

For example, if this rule/configuration is applied, it may be interpreted that the WAN UL (TX) SF linked to the DL SF(s) (PCell) carrying the (DL) signal/channel/data for the predefined usage is not (virtually) deactivated because a (UL) gap is not configured.

In another example, if a D2D TX-related (UL) gap is configured in a WAN UL (TX) SF, a linked DL SF(s) may be deactivated (virtually). If the DL SF(s) carries a (DL) signal/channel/data for a predefined usage (e.g., paging, RAR, SIB, PSS/SSS, or PSBCH), (a part or all of) the D2D RX-related (UL) gap configuration may be nullified or considered to be mis-configured. In other words, if WAN UL TX is discontinued in SF#n+4 in FDD, it is meaningless because a PUSCH/PUCCH corresponding to a UL grant or a DL allocation in SF#n cannot occur.

Therefore, it is preferred that the eNB has controllability on the afore-described ProSe gap. For example, the eNB may turn on/off the ProSe gap for each pool by UE-specific signaling, in consideration of impact on discovery performance and a WAN operation.

Accordingly, a UE may be allowed to discontinue transmission or reception of a WAN channel/signal in the ProSe gap, and may perform a discovery operation including discovery and SLSS TX/RX, like switching between carriers according to the present disclosure.

In order to efficiently support D2D discovery in a multi-carrier situation or D2D discovery on a carrier other than a PCell, if the eNB (or PCell) configures a (UL) gap in some UL SF(s) of a PCell UL CC, and a D2D TX UE performs a D2D (discovery) TX operation in another carrier (a non-PCell, a non-serving cell, or an SCell) by switching its TX chain during the (UL) gap in the foregoing method, the D2D TX UE may be defined to follow at least a part (i.e., a part or all) of rules/configurations in the following Example 9-1 or Example 9-2. The (part or all) of the rules/configurations may also be extended to D2D communication TX and/or SLSS/PSBCH TX (on another carrier, a non-PCell, or an SCell). The (part or all) of the rules/configurations may also be applied restrictively to a D2D (TX) UE having a limited TX chain capability relative to the number of carriers in which a simultaneous transmission is configured.

EXAMPLE 9-1

If a D2D TX UE performs a type-1 discovery TX operation in another carrier (non-PCell or SCell) by switching its TX chain during a (UL) gap, the D2D TX UE may be configured to select/determine type-1 discovery TX resources (based on a random or preset probability), only in consideration of resources within the (UL) gap in a type-1 D2D signal resource pool. In the case where this rule is applied, if the (UL) gap is smaller than a type-1 D2D signal resource pool (in size/period), it may be interpreted that the type-1 D2D signal resource pool is virtually redefined/configured (restrictively) with resources of the (UL) gap.

EXAMPLE 9-2

In the case where the D2D TX UE performs a type-1 discovery TX operation in another carrier (non-PCell or SCell) by switching its TX chain during a (UL) gap, if type-1 discovery TX resources that the D2D TX UE selects from a (total) type-1 D2D signal resource pool (based on a random or preset probability) are outside the (UL) gap, the D2D TX UE may be configured i) to omit the type-1 discovery TX, ii) to reselect resources (repeatedly) until the type-1 discovery TX resources that the D2D TX UE selects from the (total) type-1 D2D signal resource pool (based on the random or preset probability) fall within the (UL) gap, or ii) to apply Example 9-1.

Application of this rule may be interpreted as meaning that the D2D TX UE selects (initial) type-1 discovery TX resources based on a random or preset probability with no regard to the (UL) gap, assuming that the resources of the total type-1 D2D signal resource pool are available. In another example, if type-1 discovery TX resources that the D2D TX UE selects from the (total) type-1 D2D signal resource pool (based on the random or preset probability) are within the (UL) gap, the D2D TX UE performs a type-1 discovery Tx operation in the resources.

In another example, if the D2D TX UE should perform a type-2B/2A discovery TX operation based on resources indicated directly by an eNB (or PCell) in another carrier (non-PCell, non-serving cell, or SCell) by switching its TX chain, the eNB (or PCell) has accurate knowledge of time/frequency resources in which the D2D TX UE performs the type-2B/2A discovery TX operation, (compared to type-1 discovery Tx). Therefore, it may be defined that i) the time/frequency resources (e.g., SF), or ii) the time/frequency resources and only K (e.g., 1) SF(s) before and/after the time/frequency resources are configured/assumed to be a (UL) gap (selectively). K may be received from the eNB (or PCell) by predefined signaling (e.g., SIB, or (dedicated) RRC signaling), or may be fixed to a predetermined value (e.g., 1).

If the D2D TX UE performs the type-2B/2A discovery TX operation based on the resources indicated (directly) by the eNB (or PCell) in another carrier (non-PCell, non-serving cell, or SCell) by switching its TX chain, this may be interpreted as meaning that resource information related to the type-2B/2A discovery TX operation in another carrier (non-PCell, non-serving cell, or SCell) is configured in a cross-cell manner from the PCell (e.g., especially when it is determined that another carrier (non-PCell, non-serving cell, or SCell) is out-of-coverage from the view point of the D2D TX UE).

In another example, in the case where the D2D TX UE should perform a D2D discovery TX operation in another carrier (non-PCell, non-serving cell, or SCell) (hereinafter, referred to “OT carrier”) by switching its TX chain, if K D2D discovery TX repetitions are configured within a D2D discovery period, i) it is assumed/defined that the eNB (or PCell) does not schedule a (PCell) WAN UL TX overlapped in the time domain at least partially (i.e., partially or fully) with the K D2D discovery repetitions within the D2D discovery period, or ii) the D2D TX UE may be configured to discard scheduling information about a (PCell) WAN UL TX overlapped in the time domain (i.e., partially or fully) with the K D2D discovery repetitions within the D2D discovery period, or to consider the scheduling information to be invalid.

The former i) may be applied or valid in the case where the PCell (or eNB) knows i) D2D discovery TX time/frequency resource information (or D2D discovery TX resource pool information) and/or ii) time/frequency synchronization information, on OT carrier related to the D2D TX UE.

On the other hand, the latter ii) may be applied or valid in the case where the PCell (or eNB) has difficulty in acquiring i) D2D discovery TX time/frequency resource information (or D2D discovery TX resource pool information) and/or ii) time/frequency synchronization information, on OT_carrier related to the D2D TX UE (e.g., in the case where OT_carrier is configured (along with the PCell), for dual connectivity, and/or OT_carrier is an inter-PLMN carrier (with respect to the PCell)).

For example, in the case where the D2D TX UE should perform a D2D discovery TX operation on OT carrier by switching its TX chain, if K D2D discovery TX repetitions are configured within a D2D discovery period, the D2D TX UE may be configured to select resources (based on a random or preset probability), only in consideration of resources which are at once i) resources within a preset or signaled (UL) gap period (PCell) in a discovery resource pool (OT_carrier) and ii) resources allowing (supporting) all of the K D2D discovery repetitions.

In another example, in the case where the D2D TX UE should repeatedly perform K D2D discovery TX operations on OT_carrier by switching its TX chain, if at least a part (i.e., a part or all) of K D2D discovery TX resources that the D2D TX UE selects from a (total) D2D discovery resource pool (based on a random or preset probability) are outside a (UL) gap, the D2D TX UE may be configured i) to drop the total D2D discovery TX, ii) to reselect resources repeatedly until K D2D discovery TX resources that the D2D TX UE selects from the (total) D2D discovery resource pool (based on a random or preset probability) fall within the (UL) gap, or iii) to select resources (based on a random or preset probability), only in consideration of resources which are at once resources within a preset or signaled (UL) gap period (PCell) in a discovery resource pool (OT_carrier) and resources allowing (supporting) all of the K D2D discovery repetitions.

In another example, if a type-2B discovery TX on OT_carrier can be indicated (by the PCell), it may be configured that a (UL) gap (PCell) is implicitly considered/assumed to hop according to a type-2B discovery time hopping pattern (OT_carrier), or a PCell UL SF corresponding to the type-2B discovery time hopping pattern (OT_carrier), and K (e.g., 1) PCell UL SFs before and/or after the PCell UL SF are also considered/assumed to be a (UL) gap (PCell). K may be received from the eNB (or PCell) by predefined signaling (e.g., an SIB or (dedicated) RRC signaling) or preset to a specific value (e.g., 1).

In another example, a (serving) network (or (serving) eNB) may not have accurate information about a D2D ((TX/RX) resource) configuration on an inter-PLMN (non-primary or non-serving) carrier under an uncoordinated inter-PLMN scenario (i.e., refer to [Table 3.14]). In this case, collision(s) (or overlap) may occur between a preset (or signaled) D2D (TX/RX) resource(s)” or “WAN UL TX” on a “primary carrier” of the (serving) network (or (serving) eNB) and a “D2D (TX/RX) resource(s) in an inter-PLMN (non-primary or non-serving) carrier”, which degrades inter-PLMN D2D performance. To mitigate this problem, for example, a preset (or signaled) partial (or whole) ProSe gap (configuration information about the ProSe gap) may be configured to be randomized in time (in the time domain) according to a specific (pseudo) function (Rule #Q). Herein, for example, the ProSe gap may be interpreted as a resource area in which the (serving) network (or (serving) eNB)) allows an inter-PLMN D2D (TX/RX) operation for a (serving) D2D UE. For the convenience of description, for example, it is assumed that the ProSe gap configuration information includes offset information (i.e., referred to as “GAP_OFFSET) (applied with respect to SFN#0 of a serving carrier or a primary carrier), information about a “bitmap for a ProSe gap resource pool) (i.e., referred to as GAP_RSCBITMAP”), and information about a period (referred to as “GAP_PERIOD”). In a specific example, a (pseudo) function that randomizes part (or all) of the ProSe gap information (e.g., GAP_OFFSET (i.e., the position of the ProSe gap is time-shifted periodically), GAP_RSCBITMAP (i.e., the size of the ProSe gap resource pool is changed periodically), GAP_PERIOD (i.e., the frequency of configuring the ProSe gap is changed (periodically)) in time (in the time domain) (or on the basis of GAP_PERIOD (or a preset number of GAP_PERIODs)) may have the followings as an input value(s).

(intra- or inter-)PLMN ID

(intra-PLMN or inter-PLMN) (D2D) carrier frequency

(primary (or serving) or non-primary) (virtual/physical) cell ID

(primary (or serving) carrier) SFN (or subframe index or slot index)

(primary (or serving) carrier) DFN (D2D (sub)frame number))

UE ID or pre-signaled (or preset) random seed value

pre-signaled (or preset) GAP_OFFSET OR GAP_PERIOD (or GAP_RSCBITMAP (size))

In another example, it may be regulated that the preset (or signaled) ProSe gap hops between carriers in time (in the time domain), and the (pseudo) function that determines a hopping pattern has (a part or all of) the foregoing parameter(s) (i.e., RULE#Q) as an input value(s). For example, (part or all of) the ProSe gap configuration information applied to a carrier hopping operation may be further randomized according to RULE#Q.

Since the examples of the above-described proposed methods may be included as one of the methods for implementing the present disclosure, it is apparent that they may be considered to be a kind of proposed methods. Further, the proposed methods may be implemented independently or in combination.

It may be configured that the above-described proposed methods are applied restrictively under an FDD system environment and/or a TDD system environment.

It may be configured that the above-described proposed methods are applied restrictively to mode 2 communication and/or type 1 discovery (and/or mode 1 communication and/or type 2 discovery).

It may be configured that the above-described proposed methods are applied restrictively to a case in which a D2D RX UE receives neighbor cell-related synchronization error information of w1 related to an ‘inter-cell discovery signal’ (and/or a ‘neighbor cell discovery signal’).

It may be configured that the above-described proposed methods are applied restrictively to at least one of an in-coverage D2D UE, an out-coverage D2D UE, an RRC Connected D2D UE, or an RRC Idle D2D UE.

Further, it may be configured that the above-described proposed methods are applied respectively to a D2D UE performing only a D2D discovery (transmission/reception) operation (and/or a D2D communication (TX/RX) operation).

Further, it may be configured that the above-described proposed methods are applied respectively to a scenario in which only D2D discovery is supported/configured (and/or a scenario in which only D2D communication is supported/configured).

Further, the CEILING(X) function (i.e., a function of deriving a smallest integer equal to or greater than X) may be replaced with the FLOOR(X) function (i.e., a function of deriving a larges integer equal to or greater than X).

Further, it may be configured that the above-described proposed methods are applied respectively to an SHRXCH_D2D RX UE (and/or an SRXCH_D2D RX UE).

Further, it may be configured that the above-described proposed methods are applied respectively to CA or non-CA.

Further, it may be configured that the above-described proposed methods are applied respectively to a case in which a D2D discovery signal reception operation is performed in another (UL) carrier of an inter-frequency, and/or a case in which a D2D discovery signal reception operation is performed in another PLMN (UL) carrier based on inter-PLMN.

FIG. 17 is a block diagram of a BS and a UE which are applicable to an embodiment of the present disclosure.

If a relay is included in a wireless communication system, communication is conducted between a BS and the relay via a backhaul link, and communication is conducted between the relay and a UE via an access link. Hence, the BS or the UE illustrated in FIG. 17 may be replaced with the relay under circumstances.

Referring to FIG. 17, a wireless communication system includes a BS 110 and a UE 120. The BS 110 includes a processor 112, a memory 114, and an RF unit 116. The processor 112 may be configured to perform the procedures and/or methods proposed in the present disclosure. The memory 114 is connected to the processor 112 and stores various kinds of information related to an operation of the processor 112. The RF unit 116 is connected to the processor 112 and transmits and/or receives a radio signal. The UE 120 includes a processor 122, a memory 124 and an RF unit 126. The processor 122 may be configured to perform the procedures and/or methods proposed in the present disclosure. The memory 124 is connected to the processor 122 and stores various kinds of information related to an operation of the processor 122. The RF unit 126 is connected to the processor 122 and transmits and/or receives a radio signal. The BS 110 and/or the UE 120 may have a single antenna or multiple antennas.

The embodiments of the present disclosure described above are combinations of elements and features of the present disclosure in specific forms. The elements or features may be considered selective unless otherwise mentioned. Each element or feature may be practiced without being combined with other elements or features. Further, an embodiment of the present disclosure may be constructed by combining parts of the elements and/or features. Operation orders described in embodiments of the present disclosure may be rearranged. Some constructions or elements of any one embodiment may be included in another embodiment and may be replaced with corresponding constructions or features of another embodiment. It is obvious to those skilled in the art that claims that are not explicitly cited in each other in the appended claims may be presented in combination as an embodiment of the present disclosure or included as a new claim by a subsequent amendment after the application is filed.

In the embodiments of the present disclosure, a specific operation described as being performed by a BS may be performed by an upper node of the BS. Namely, it is apparent that, in a network comprised of a plurality of network nodes including a BS, various operations performed for communication with a UE may be performed by the BS, or network nodes other than the BS. The term, BS may be replaced with a fixed station, a Node B, an eNode B (eNB), an access point, and so on.

Embodiments of the present disclosure may be achieved by various means, for example, hardware, firmware, software, or a combination thereof In a hardware configuration, an embodiment of the present disclosure may be achieved by one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, microcontrollers, microprocessors, and so on.

In a firmware or software configuration, an embodiment of the present disclosure may be implemented in the form of a module, a procedure, a function, and so on performing the above-described functions or operations. A software code may be stored in a memory unit and executed by a processor.

The memory unit may be located at the interior or exterior of the processor and may transmit and receive data to and from the processor via various known means.

Those skilled in the art will appreciate that the present disclosure may be carried out in other specific ways than those set forth herein without departing from the spirit and essential characteristics of the present disclosure. The above embodiments are therefore to be construed in all aspects as illustrative and not restrictive. The scope of the disclosure should be determined by the appended claims and their legal equivalents, not by the above description, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.

INDUSTRIAL APPLICABILITY

While the method and apparatus for transmitting and receiving a D2D signal in a wireless communication system have been described in the context of a 3 GPP LTE system, they are also applicable to various wireless communication systems other than the 3GPP LTE system. 

1-6. (canceled)
 7. A method for transmitting a device-to-device (D2D) discovery signal by a user equipment (UE) in a wireless communication system supporting carrier aggregation (CA), the method comprising: selecting a reference cell from among a primary cell or secondary cells according to whether or not D2D signals are to be transmitted by using, respectively, the primary cell or the secondary cells; selecting a reference signal received power (RSRP)-based resource pool from among multiple resource pools based on the selected reference cell; and transmitting the D2D signals by using the selected RSRP-based resource pool.
 8. The method according to claim 7, wherein each of the multiple resource pools is configured by higher layer signaling.
 9. The method according to claim 7, further comprising: if the UE is configured with a discovery transmission reference cell for the D2D discovery signals transmission, using a reference cell indicated by the discovery transmission reference cell.
 10. The method according to claim 7, further comprising: using a serving cell as the reference cell in a radio resource control (RRC) idle state.
 11. The method according to claim 7, further comprising: using the primary cell as the reference cell in a radio resource control (RRC) connected state.
 12. A user equipment (UE) for transmitting a device-to-device (D2D) discovery signal in a wireless communication system supporting carrier aggregation (CA), the UE comprising: a radio frequency (RF) transceiver; and a processor, wherein the processor is configured to: select a reference cell from among a primary cell or secondary cells according to whether or not D2D signals are to be transmitted by using, respectively, the primary cell or the secondary cells, select a reference signal received power (RSRP)-based resource pool from among multiple resource pools based on the selected reference cell, and control the RF transceiver to transmit the D2D signals by using the selected RSRP-based resource pool.
 13. The UE according to claim 12, wherein each of the multiple resource pools is configured by higher layer signaling.
 14. The UE according to claim 12, wherein, if the UE is configured with a discovery transmission reference cell for the D2D discovery signals transmission, the processor is further configured to use a reference cell indicated by the discovery transmission reference cell.
 15. The UE according to claim 12, wherein the processor is further configured to use a serving cell as the reference cell in a radio resource control (RRC) idle state.
 16. The UE according to claim 12, wherein the processor is further configured to use the primary cell as the reference cell in a radio resource control (RRC) connected state. 